重费米子超导体CeCoIn₅和CeIrIn₅中的Cd掺杂研究

浙江大学  路欣

Ce-115系列化合物是当前最受关注的一类重费米子材料，表现出异常丰富的物理性质。与其它关联电子材料相比，重费米子化合物的基态性质常可通过掺杂、压力或者磁场进行调控。CeCoIn₅和CeIrIn₅在常压下即为重费米子超导体，但是1.0%左右的Cd掺杂就可以破坏超导，诱发反铁磁长程序，其物理机理仍存在激烈争论。1% Cd掺杂的CeCoIn₅或者CeIrIn₅样品在不同压力条件下的低温电阻和比热测量表明它的反铁磁相逐渐被压力抑制，但是没有呈现相关的反铁磁量子临界现象，而重新出现的超导态与压力下相关纯样品的超导性质非常类似[1,2]。而量子临界点附近的自旋液滴模型可以成功解释相关的实验现象： CeCoIn₅和CeIrIn₅在常压下都处于反铁磁量子临界点附近，Cd杂质周围由于临界涨落而凝聚成较大的自旋液滴，它们如果在空间中交叠就会形成反铁磁长程序；而压力下的CeIrIn₅ 和CeCoIn₅会远离其量子临界点，临界涨落的减弱会导致自旋液滴尺寸减小，反铁磁长程序因退相干而最终消失。该发现揭示了量子临界点附近的一般规律，即少量的杂质即会产生非均匀电子态从而影响物质的本征态。

 

参考文献：

[1] S. Seo⁺, Xin Lu⁺, J.-X. Zhu, R. R. Urbano, N. Curro, E. D. Bauer, V. A. Sidorov, L. D. Pham, Tuson Park*, Z. Fisk and J. D. Thompson*, Nature Physics 10, 120 (2014);

[2] Y. Chen, W. B. Jiang, C. Y. Guo, F. Ronning, E. D. Bauer, Tuson Park, H. Q. Yuan, Z. Fisk, J. D. Thompson and Xin Lu*，Phys. Rev. Lett. 114, 146403 (2015) 

 

 

Our recent progress of high pressure studies on superconductivity

 

Liling Sun

 

¹Institute of Physics and Beijing National Laboratory for Condensed Matter Physics,Chinese Academy of Sciences, Beijing 100190, China

²Collaborative Innovation Center of Quantum Matter, Beijing, 100190, China

       Superconductivity of materials has been thought to be determined by the states of charge, orbital and spin degrees of freedom, as well as lattice. These states can be manipulated by control parameters including pressure, magnetic field and chemical doping. Among these parameters, pressure is a 'clean' way for tuning the crystal and electronic structures. In this talk, our progress on high pressure studies of WTe₂, a nonmagnetic compound with large magnetoresistance (LMR) at ambient pressure [Ali et al, Nature 2014], is reported. We demonstrate the suppression of the LMR and emergence of superconductivity in pressurized WTe₂ with the high-pressure experimental results from high-pressure synchrotron X-ray diffraction, electrical resistance, magnetoresistance, and ac magnetic susceptibility measurements. In addition, high pressure behaviors of new discovered superconductor (K,Rb)₂Cr₃As₃ is reported.

 

In collaboration with Prof. Guangming Zhang, Prof. Youguo Shi and Prof. Guanghan Cao

 

 

Single-mode Polariton Laser in a Designable Microcavity
Bo Zhang¹, Zhaorong Wang¹, Seonghoon Kim¹, Sebastian Brodbeck², Christian Schneider²,
Martin Kamp², Sven HÖfling², Hui Deng¹
1. University of Michigan, Ann Arbor, Michigan, 48109, USA.

2. University of Wuerzburg, Wuerzburg, D-97074, Germany

dengh@umich.edu

We demonstrate strong coupling and a coherent polariton laser in a microcavity consisting of a sub-wavelength grating in the top mirror. The designable grating mirror allows 3D confinement, polarization selectivity, and dispersion engineering by design.

References：
[1] H. Deng, H. Haug, and Y. Yamamoto, Rev. Mod. Phys. 82, 1489 (2010).
[2] I. Carusotto and C. Ciuti, Rev. Mod. Phys. 85, 299 (2013).
[3] B. Zhang, Z. Wang, S. Brodbeck, C. Schneider, M. Kamp, S. Höfling, and H. Deng, Light Sci. Appl. 3, e135 (2014).
[4] C. F. R. Mateus, M. C. Y. Huang, Y. Deng, A. R. Neureuther, and C. J. Chang-Hasnain, IEEE Photonics Technol. Lett. 16, 518 (2004).
[5] Z. Wang, B. Zhang, and H. Deng, Phys. Rev. Lett. 114, 073601 (2015).
[6] B. Zhang, S. Brodbeck, Z. Wang, M. Kamp, C. Schneider, S. Höfling, and H. Deng, Appl. Phys. Lett. 106, 051104 (2015).
[7] L. K. Thomsen and H. M. Wiseman, Phys. Rev. A 65, 063607 (2002).
 

计算凝聚态物理分会 “ 凝聚物质的激发态与动力学理论和实验 ”专题讨论会邀请报告

Many-body Interactions in 1D Carbon Nanotubes System

刘开辉研究员（北京大学 物理学院）

When the characteristic length of a material shrinks to 1 nm scale, many distinct physical phenomena, such as quantum confinement, enhanced many-body interactions and strong van der Waals inter-material couplings, will appear. To investigate these fascinating low-dimensional physics, it will be very appealing to have a tool to quantitatively link the atomic structures to the physical properties of these very small nanomaterials. In this talk, I will introduce our recently developed in-situ TEM + high-sensitive ultrafast nanooptics technique [1,2], which combines capability of structural characterization in TEM and property characterization in nano-optics on the same individual nanomaterials. Several examples of using this technique to study the 1D physics in carbon nanotube system will be demonstrated [3-6].

 

References:

First-principles investigation on ultrafast laser and nano-materials

Hong Zhang

College of Physical Science and Technology, Sichuan University, China

 

I will present the first-principles investigation on ultrafast laser pulse which induces dynamics in molecules encapsulated by a nanotube. A strong laser pulse induces a giant bond-stretch of an HCl molecule inside both C and BN nanotubes. Depending on the initial orientation of the HCl molecule, the subsequent laser-induced dynamics is different. We also observed a radial motion of the nanotube and vacancies appear on the tube wall when the HCl is perpendicular to tube axis, the disintegration of HCl molecules took place when their molecular axis tilted to tube axis. Furthermore, I will also mention how the situation is when C2H2 encapsulated inside nanotube. These simulations are important to analyze light-induced nanochemistry and manipulation of nanostructures encapsulated in organic and inorganic nanotubes.

References:

1. H. Zhang, Y. Miyamoto, A. Rubio, Phys. Rev. Lett., 109, 265505, 2012

2. H. Zhang, et al, Appl. Phys. Lett. 95, 053109, 2009

3. H. Zhang, et al, Phys. Rev. B 85, 201409(R) , 2012

4. H. Zhang, et al, Phys. Rev. Lett., submitted, 2014

First-Principles Models of Thermal Conductivity of Lower Mantle Minerals at Extreme High Pressure-Temperature Conditions

Jianjun Dong

Department of Physics, Auburn University, USA

 

地幔矿物在超高温和超高压条件下的热输运性质是研究地球内部过程及演化的重要参数。但是由于实验条件的局限以及地幔矿物组成和结构的复杂性，目前对这些关键的材料性质的估计仍然存在很大的不确定性。更重要的是目前估计的热导率的最高值和最低值分别支持截然不同的地球演化理论。在今天的报告中， 我首先介绍我们近来在声子散射和弛豫时间的第一性计算方面的进展［1，2，3］。为了准确的预测复杂的地幔矿物的热导率，我们发展了高效率，大规模并行计算的数值方法， 计算了钙钛矿(MgSiO₃ perovskite, 20 atoms per unit-cell)和氧化物(MgO)在整个布里渊区中的所有声子弛豫时间，并且考虑了铁(Fe)原子对镁(Mg)原子的参杂效应, 以及氧化物和钙钛矿的复合材料平均。基于这些第一性计算结果，我们估测地球在核幔边界区域的平均总热导率 介于2.5-3.5 W/m/K。接下来， 我将介绍我们最新的collective 弛豫时间理论［4］，并在这个理论的框架内讨论Peierls 的kinetic 声子输运公式和Boltzmann的 声子输运公式 在不同温度下的差别。 最后， 我将讨论我们对高温条件下红外声子和红外光子耦合所产生的diffusive bulk 声子极化子的实验和理论研究数据， 以及相应的一个新的高温条件下热导机制 ［5］。

 

* The reported research work is financially supported by NSF grants awarded to Dong (EAR-0757847 and EAR-1346961).

 

[1] X. Tang and J. Dong, Phys. Earth Planet. Int 174, 33 (2009).

[2] X. Tang and J. Dong, Proc. Natl. Acad. Sci., U.S.A. 107, 4539 (2010).

[3] X. Tang et al. Geophy. Rev. Lett. 41, 2746 (2014).

[4] J. Dong et al, to be submitted to Phys. Rev. B.

[5] A.M. Hoffmeister, J. Dong, and J.M. Branlund, J. Appl. Phys. 115, 163517(2014).

 

 

 

 

1. Y. Qiu, W. Bao*, Q. Huang et al., Phys. Rev. Lett. 101, 257002 (2008).

2. Q. Huang, Y. Qiu, W. Bao* et al., Phys. Rev. Lett. 101, 257003 (2008).

3. W. Bao*,  Y. Qiu, Q. Huang et al., Phys. Rev. Lett. 102, 247001 (2009).

4. W. Bao*, Q. Huang, G. Chen et al., Chinese Phys. Lett. 28, 086104 (2011).

5. W. Bao, Chinese Phys. B 22, 087405 (2013).

6. W. Lu et al., Phys. Rev. B 84, 155107 (2011); W. G. Yin et al., ibid. 86, 081106 (2011).

7. W. Bao*, G.-N. Li, Q. Huang et al., Chinese Phys. Lett. 30, 027402 (2013).

8. L. Sun et al., Nature 483, 67 (2012); J. Guo et al., Phys. Rev. Lett. 108, 197001 (2012); M. Gooch et al., Phys. Rev. B 84, 184517 (2011); V. Ksenofontov et al., ibid 85, 214519 (2012).

9. F. Ye, W. Bao*, S. Chi et al., Chinese Phys. Lett. 31, 127401 (2014).

10. W. Bao, J. Phys. Cond. Matter 27, 023201 (2015).

Electric-field control of magnetism in multiferroic heterostructures

Y. G. Zhao

Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084, China

Collaborative Innovation Center of Quantum Matter, Beijing 100084, China

 

        With the fast development of information storage, exploiting new concepts for dense, fast, and non-volatile random access memory with reduced energy consumption is a significant and challenging task. To realize this goal, electric-field control of magnetism is crucial. In this regard, multiferroic materials are important and have attracted much attention due to their interesting new physics and potentials for exploring novel multifunctional devices [1]. In the multiferroic materials, electric polarization can be tuned by external magnetic fields or vice versa. This magnetoelectric (ME) effect originates from the coupling of the magnetic and ferroelectric orders. However, single-phase multiferroic materials are rare and the multiferroic heterostructures, composed of ferromagnetic (FM) and ferroelectric (FE) materials, provide an alternative way for exploring the ME coupling effect. One of the key issues in the study of the FM/FE heterostructures is the control of magnetism via electric fields, which is essential for the new generation information storage technology. We have carried out electric-field control of magnetism in different FM/FE heterostructures [2], such as Co₄₀Fe₄₀B₂₀(CoFeB)/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃(PMNT) and magnetic tunnel junction (MTJ) on PMNT, etc. Our work demonstrates the interesting new physics and potential applications of electric-field control of magnetism in the FM/FE multiferroic heterostructures.

 

References

1. W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 442,759 (2006); R. Ramesh and N. A. Spaldin, Nature Mater. 6, 21 (2007).
2. S. Zhang et al., Phys. Rev. Lett. 108, 137203 (2012); S. Zhang et al., Scientific Reports 4, 3727 (2014); L. F. Yang et al., Scientific Reports 4, 4591 (2014); P. S. Li et al., Adv. Mater. 46, 2340 (2014); Q. P. Chen et al., unpublished; A. T. Chen et al., unpublished, Y. Liu et al., unpublished.

 

Multiferroics, displaying magnetic, polar, and elastic order parameters simultaneously, have attracted numerous research interests. In order to speed up realistic applications of multiferroics, it is highly desirable to predict new high-performance multiferroics. We predicted that hexaferrite BaFe₁₂O₁₉ may be the first example of multiferroic materials that displays antiferroelectricity [1]. The antiferroelectricity in this system is “geometrically frustrated” by the underlying hexagonal structure. The antiferroelectric state can also be driven by an external electric field into a metastable ferroelectric state. This property together with its room temperature ferrimagnetism may be exploited for applications as multiple-state memory devices. Besides, we propose the concept of a new type of multiferroics, namely, “asymmetric multiferroic” [2]. In asymmetric multiferroics, two locally stable ferroelectric states are not symmetrically equivalent, leading to different magnetic properties between them. We further predict that a Fe-Cr-Mo superlattice with the LiNbO₃-type structure is an asymmetric multiferroic. Finally, we predict that double perovskite Zn₂FeOsO₆ is a new multiferroic with properties superior to BiFeO₃. First, there are strong ferroelectricity and strong ferrimagnetism at room temperature in Zn₂FeOsO₆. Second, the easy-plane of the spontaneous magnetization can be switched by an external electric field, evidencing the strong magnetoelectric coupling existing in this system. Our results suggest that ferrimagnetic 3d-5d double perovskite may therefore be used to achieve voltage control of magnetism in future spintronic devices [3].

References:

[1] P. S. Wang and H. J. Xiang*, Phys. Rev. X 4, 011035 (2014).

[2] X. Z. Lu and H. J. Xiang*, Phys. Rev. B 90, 104409 (2014).

[3] P. S. Wang, W. Ren, L. Bellaiche, and H. J. Xiang*, Phys. Rev. Lett. 114, 147204 (2015).

The possibility of using exchange interactions to manipulate the spin state of an antiferromagnetic nanostructure is explored with \textit{ab initio} calculations. Taking M \chem{(M=Mn, Fe, Co)} mono-atomic chains supported on \chem{Cu_{2}N} islands on a \chem{Cu(100)} surface as a model system, it is demonstrated that two indistinguishable N\'eel states of an antiferromagnetic chain can be tuned into a preferred state by coupling the chain to a magnetic STM tip. The magnitude and direction of the antiferromagnetic chain's intrisic anisotropy can be tailored with exchange coupling by varying the tip-chain separation.

We have performed a systematic study on a series of low dimensional TiO₂ nanostructures under density functional theory (DFT) methods. The geometries, stabilities, growth mechanism, and electronic structures of 1D chain, 2D ring, 2D ring array and 3D network of TiO₂ nanostructures are analyzed. Based on the Ti₂O₄ building unit, a series of 1D TiO₂ nano chains and rings can be built. Furthermore, 2D ring array and 3D network nanostructures can be constructed from 1D chains and rings. Among non-periodic TiO₂ chain and ring structures, one series of ring structures are found to be more stable. The geometry model of the 2D ring arrays and 3D network structures in this work has provided a theoretical understanding on the structure information in experiments. Based on these semiconductive low dimensional structures, moreover, it can help to understand and design new hierarchical TiO₂ nanostructure in the future.

In this talk, I will introduce several types of (non-Abelian) gauge fields in graphene and show their effects on electronic properties of graphene. With the help of the gauge fields generated by magnetic fields, we realize Landau quantization in graphene monolayer, Bernal bilayer and trilayer. In a gapped graphene monolayer, we direct image the two-component Dirac-Landau levels in atomic resolution. In strained graphene, lattice deformations induce valley-dependent gauge (pseudomagnetic) fields. I will show direct and compelling evidence for electron confinement and valley splitting in grapheme monolayer due to the coexistence of both the magnetic field and the pseudomagnetic field.

Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. In this talk, I will also show the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers. We direct demonstrate that the non-Abelian gauge potentials in twisted graphene bilayers confine low-energy electrons into a triangular array of quantum dots following the modulation of the Moiré patterns.

In the higher Landau levels (N >0) a reentrant integer quantum Hall effect (RIQHE) state, which resides at fractional filling factors but exhibits integer Hall plateaus, has been previously observed and studied extensively. The nonlinear dynamics of the RIQHE were measured by microwave resonance, with the results consistent with an electronic bubble phase pinned by impurities.We have carried out depinning experiments on the N = 2 bubble phases by using Corbino geometry, where depinning threshold values have been systematically measured as a function of magnetic fields and temperatures. Domain sizes and pinning potential of the bubble phases have been estimated from the nonlinear transport data.

本文采用系统的密度泛函微扰理论（DFPT）计算，确定了单层h-BN在双轴拉伸下发生结构相变所对应的声子模式，该模式不同于人们熟知的石墨烯K1软模，其中B原子和N原子沿不同方向呈现三角振动，相应的振动模式是扭曲的。进行电荷掺杂后，单层h-BN的理想强度得到了显著增强。但对于重掺杂的单层h-BN，其软模不再是K1模，出现虚频的临界波矢也从K点移至别的对称点。扭曲的K1软模既具有拉曼活性又具有红外活性，因此跟踪实验光谱中该模式的演变可以很好地解释单层h-BN的力学失效机制。

Quantum coherence is the key ingredient of many exotic states of matter. Epitomes are the superconductivity and the Bose-Einstein condensation. Interestingly, much of the physics governing the aforementioned states of matter finds its application in a quantum Hall state formed at high magnetic fields. In a double-quantum well GaAs/AlGaAs heterostructure, interlayer coherence emerges at a total Landau level filling of one, provided that the layers are closely spaced. This coherence manifests itself as a zero Hall resistance plateau, a nearly perfect Coulomb drag and an interlayer Josephson current. In this talk, a brief review of the recent progress on the quantum Hall bilayer will be presented. The enhancement of Josephson current in bilayer samples with reduced interlayer distances will be discussed. As the interlayer coupling increases, we observed a breakdown of uniform coherence. This phenomenon likely signals a transition of the bilayer into a fluctuation dominant regime.

    我们对高质量的HgCr₂Se₄单晶样品进行了磁化率、电子输运和安德烈夫（Andreev）反射谱等方面的研究。温度足够低时，单个晶胞的饱和磁矩对应于整数个玻尔磁子（6 μ_B/formular unit）。对HgCr₂Se₄/Pb安德烈夫平面结的微分电导谱利用修正的Blonder-Tinkham-Klapwijk (BTK)理论进行拟合，得到了高达97%的自旋极化率。小偏压下的微分电导对偏压的抛物型依赖关系与BTK理论明显偏离。零偏压电导的温度依赖关系与最近提出的基于半极性金属/超导体界面处自旋活跃散射的理论模型相符合。我们的实验结果说明n型HgCr₂Se₄的基态是半极性金属性铁磁体，这和同时预言了非掺杂HgCr₂Se₄是磁性Weyl半金属的第一性原理计算结果基本一致。

    本工作与中国科学院物理研究所的翁红明、戴希、方忠、山东大学的颜世申和Florida State University的熊鹏合作完成，受基金委、科技部和中科院资助。

参考文献： T. Guan et al., arXiv: 1503.03190.

钙钛矿型LaAlO₃/SrMnO₃和LaAlO₃/EuTiO₃异质界面电子结构重构的第一性原理研究

雎胜

苏州大学物理系

近年来，LaAlO₃/SrTiO₃异质界面电子结构重构及其带来的二维电子气引起了科研工作者的广泛兴趣。这里，基于密度泛函理论，我们设计并研究了两种钙钛矿型氧化物异质界面电子结构和磁电性质。我们在LaAlO₃/SrMnO₃[1]和LaAlO₃/EuTiO₃[2]超晶格中分别发现了n型和p型半金属电子结构。前者来源于Mn⁴⁺离子上e_g电子的部分占据及其带来的双交换半金属铁磁性；而后者来源于Eu²⁺离子上4f电子的局域性和较大的自旋辟裂。另外，我们发现可以通过改变极性层LaAlO₃的厚度来调制体系能带结构以获得绝缘性到金属性的转变，以及磁电阻效应和完全自旋极化电子-空穴对。这些结果为实现基于氧化物的自旋电子学器件提供了一些材料结构的设计方案。

 

1. F. Hou, T.Y. Cai, S. Ju, and M. R. Shen, ACS Nano 6, 8552 (2012).
2. H. S. Lu, T. Y. Cai, S. Ju, and C. D. Gong, Phys. Rev. Applied 3, 034011 (2015).

to be inputted ...

Candidates for correlated topological insulators, originated from the spin-orbit coupling as well as Hubbard type correlation, are expected in the (111) bilayer of perovskite-structural transition-metal oxides.[1] Based on the first-principles calculation and tight-binding model, the electronic structure of a LaMnO3 (111) bilayer sandwiched in LaScO3 barriers has been investigated. For the ideal
undistorted perovskite structure, the quadratic band touching at the Gamma point and the Dirac cone at the K point are found. Naturally, the Fermi energy of LaMnO3 (111) bilayer just stays at the Dirac point, rendering a semi-metal (graphene-like) which is also a half-metal (different from graphene nor previous studied LaNiO3 (111) bilayer). The bands touching at the Dirac cone and the Gamma point can be opened by the spin-orbit coupling, giving rise to nontrivial topological bands corresponding to the (quantized) anomalous Hall effect. For the realistic orthorhombic distorted lattice, the Dirac point moves away from the K point to Gamma point gradually with increasing Hubbard repulsion (or equivalent Jahn-Teller distortion in the tight-binding model). Finally, a Mott gap opens at the Gamma point, establishing a phase boundary between the Mott insulator and topological magnetic insulator. Our calculation finds that the gap opened by spin-orbit coupling is much smaller in the orthorhombic distorted lattice (1.7 meV) than the undistorted one (11 meV). Therefore, to suppress the lattice distortion can be helpful to enhance the robustness of topological phase in perovskite (111) bilayers.[2]

Reference

[1] D. Xiao, W. Zhu, Y. Ran, N. Nagaosa, and S. Okamoto, Nat. Commun. 2, 596 (2011).

[2] Y. K. Weng, X. Huang, Y. G. Yao and S. Dong, Phys. Rev. B, submitted.

Interfaces in oxide heterostructures always provide a fertile ground for emergent properties. Charge transfer from a high energy band to a low energy opponent is naturally expected, as occurring in semiconductor p-n junctions. In this study, several exceptional physical phenomena have been predicted in (YFeO3)n/(YTiO3)n superlattices. First, the charge transfer between these Mott insulators is in opposition to the intuitive band alignment scenario. Second, hybrid ferroelectricity with a moderate polarization is generated in the n = 2 magnetic superlattice. Furthermore, the ferroelectric-type distortion can persist even if the (ABO3)2/(AB'O3)2 system turns out to be metallic, rending possible metallic ferroelectricity. [1]

Refrence
[1] Huimin Zhang, Yakui Weng, Xiaoyan Yao, and Shuai Dong, Phys. Rev. B 91, 195145 (2015).

随着自旋电子学对高密度信息存储和高速信息处理能力要求的日益提高，纳米尺度磁性结构上超快自旋动力学成为了近年来的研究热点之一。本报告采用第一性原理计算，研究磁性团簇和配合物上基于Λ进程的超快磁性操控（如退磁，自旋翻转，自旋转移），以实现结构潜在的自旋逻辑功能，从而为纳米尺度功能器件的设计提供理论指导和依据，推动分子自旋电子学器件的应用和发展。

Kondo lattice systems still puzzle researchers since 1970s. A particular challenge is to define the localized or itinerant nature of f electrons. Usually the concentrated array of localized moments leads to the ordered state, while the Kondo-screened f electrons reveal Fermi-liquid behaviors. It’s an open question whether the f electrons show the localized or itinerant properties, respectively or at the same time? In this work, we use Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Angle-Resolved Photoemission Spectroscopy (ARPES) to investigate the electronic structures of antiferromagnetic (AFM) heavy fermion compound CeIn₃. We find a gaplike decrease in the tunneling conductance at the Fermi level below 50K and an explanation is given. On further cooling, another additional gap caused by AFM transition emerges. In addition, a low energy excitation above Fermi level is visible below 20K and sustains into the AFM state. Except for the STM results, ARPES results also confirm the effect approaching the AFM transition temperature. These results suggests a possibility that even at the large moment ordered state, the itinerant component of f electrons still contributes to the Fermi surface and transport properties.

横向过渡金属硫族化合物（TMDs）超晶结构的电子结构及其性质跟材料的结构特征有很大关系。在量子限制效应、应力和内建电场的共同作用下，其表现出丰富的性质特征及巨大的应用潜力。例如，随着中间层厚度的增加，体系的带隙可以连续变小。在有的超晶格结构中，量子阱有可能实现。而且，在以上三点的共同作用下，有的超晶格体系可能实现量子拓扑绝缘态。此外，这种结构中的valleytronics性质也是一个研究兴趣。

参考文献:

[1] X.-Q. Zhang, C.-H. Lin, Y.-W. Tseng, K.-H. Huang and Y.-H. Lee, Nano Lett., 2015, 15, 410-415.

[2] X. Duan, C. Wang, J. C. Shaw, R. Cheng, Y. Chen, H. Li, X. Wu, Y. Tang, Q.  Zhang, A. Pan, J. Jiang, R. Yu, Y. Huang and X. Duan, Nature Nanotechnol., 2014, 9, 1024-1030.

[3] C. Huang, S. Wu, A. M. Sanchez, J. J. P. Peters, R. Beanland, J. S. Ross, P. Rivera, W. Yao, D. H. Cobden and X. Xu, Nature Mater., 2014, 13, 1096-1011.

[4] Y. Gong, J. Lin, X. Wang, G. Shi, S. Lei, Z. Lin, X. Zou, G. Ye, R. Vajtai, B. I. Yakobson, H. Terrones, M. Terrones, B. K. Tay, J. Lou, S. T. Pantelides, Z. Liu, W. Zhou and P. M. Ajayan, Nature Mater., 2014, 13, 1135-1142.

[5] G. D. Duesberg, Nature Mater., 2014, 13, 1075-1076.S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molnar, M. L. Roukes, A. Y. Chtchelkanova, and D. M. Treger, Science 294, 1488 (2001).

        利用平衡方程方法我们系统地研究了强磁场作用下单层二硫化钼中的线性磁输运性质。低温时，在带内杂质散射情况下，我们得到了明显的伴随着拍频模式的Shubnikov de Haas振荡现象，其中拍频波形来源于单层二硫化钼较大的自旋轨道耦合。特别地，Shubnikov de Haas振荡随磁场的倒数的振荡周期在强磁场时会加倍，这个起源于振荡高阶项。带间杂质情况下，Shubnikov de Haas振荡在强磁场下会出现一个区域电阻很小的现象。随着温度的升高，振荡的幅度减小，但是波包在较高的温度下仍旧存在，这一点和带内杂质散射的情况完全不一样。在温度高到Shubnikov de Haas振荡几乎完全被抑制时，我们还观察到了光学声子和声学声子导致的磁声共振效应。其中光学声子主要来源于单极和Fröhlich耦合，而声学声子主要来源于谷内横向和纵向声学声子。对于光学声子导致的磁声共振，我们还得到了它的拍频行为。

 

 

参考文献:

[1] K. Kaasbjerg, K. S. Thygesen, and K. W. Jacobsen, Phys. Rev. B 85, 115317 (2012).

[2] X. Li, J. T. Mullen, Z. Jin, K. M. Borysenko, M. B. Nardelli, and K. W. Kim, Phys. Rev. B 87, 115418 (2013).

[3] C. M. Wang and X. L. Lei, J. Phys.: Condens. Matter 26, 
235801 (2014).

[4] X. L. Lei, Phys. Rev. B 77, 205309 (2008).

[5] T. Cai, S. A. Yang, X. Li, F. Zhang, J. Shi, W. Yao, and 
Q. Niu, Phys. Rev. B 88, 115140 (2013).

[6] F. Rose, M. O. Goerbig, and F. Pi ́echon, Phys. Rev. B 88, 
125438 (2013).

[7] X. Zhou, Y. Liu, M. Zhou, H. Shao, and G. Zhou, Appl. 
Phys. Express 7, 021201 (2014). 

上个世纪70年代，人们曾经仔细的研究了石墨层间不同材料的插层。最近随着新型二维材料的涌现，层间插层研究又成为了一个新的研究热点【1-4】。这里我将讨论我们最近关于锂离子【2-3】和钠离子【4】在石墨烯【2】，二硫化钼【3】和还原氧化石墨【4】等二维材料里的插层研究成果。我们采用电化学可控插层技术，从光学和电学的角度实时观察了这些二维材料对插入离子浓度的响应。这些研究成果在材料和能源领域都有着广阔的应用前景

【1】“Gate-tunable Phase Transitions in Thin Flakes of 1T-TaS2” Yijun Yu, Fangyuan Yang, Xiu Fang Lu, Ya Jun Yan, Y. H. Cho, Liguo Ma, Xiaohai Niu, Sejoong Kim, Young-Woo Son, Donglai Feng, Shiyan Li, Sang-Wook Cheong, Xian Hui Chen and Yuanbo Zhang, Nature Nanotechnology 10, 270–276 (2015)

【2】”Approaching the limits of transparency and conductivity in graphitic materials through lithium intercalation” Wenzhong Bao, Jiayu Wan, Xiaogang Han, Xinghan Cai, Hongli Zhu, Dohun Kim, Dakang Ma, Yunlu Xu, Jeremy N Munday, H Dennis Drew, Michael S Fuhrer, Liangbing Hu, Nature Communications, 2014, doi:10.1038/ncomms5224

【3】”In Situ Investigations of Li-MoS2 with Planar Batteries”, Jiayu Wan, Wenzhong Bao, Yang Liu, Jiaqi Dai, Fei Shen, Lihui Zhou, Xinghan Cai, Daniel Urban, Yuanyuan Li, Katherine Jungjohann, Michael S Fuhrer, Liangbing Hu, Advanced Energy Materials, 2015, DOI: 10.1002/aenm.201401742

【4】”Sodium-Ion Intercalated Transparent Conductors with Printed Reduced Graphene Oxide Networks” Jiayu Wan , Feng Gu , Wenzhong Bao , Jiaqi Dai , Fei Shen , Wei Luo , Xiaogang Han , Daniel Urban , and Liangbing Hu, Nano Letters, 2015, DOI: 10.1021/acs.nanolett.5b00300 

 

 

The advent and development of nanomaterials have been an increasing interest to investigate the quantum mechanics confined to curved surface. For a quantum particle bounded on curved surface, the well-known geometric potential presences in the surface quantum mechanics. If the quantum particle is considered spin, not only the well-known geometric potential but also the spin connection geometric potential will appear in the surface quantum mechanics. It is worth noticing, for a quantum particle with spin, that the surface quantum equation just is relevant to the normal part of spin. In semiconductor device technology, the surface thickness may be important to the quantum dynamics. In order to facilitate we clearly refine the fundamental framework for the thin-layer quantization formalism. We further enhance the formalism to consider the effects of the surface thickness on the constrained system.

在石墨烯异质结中，垂直异质结界面入射的电子，可以完全透射穿过高的势垒区，这一现象是著名的Klein隧穿效应[1-2]，也是狄拉克费米子体系的典型特征之一。由于Klein隧穿效应的存在，很难用电学方法限制无质量的狄拉克费米子[3-4]。近年来，研究发现非均匀的磁场会破坏Klein隧穿，从而调控石墨烯体系的输运性质[5-6]。在石墨烯器件中，实验上也观察到库伦阻塞和量子限制效应[7]。

我们研究了石墨烯p-n结，p-n-p结在磁场作用下的输运性质[8]。我们发现以入射角为变量的透射曲线，在磁场作用下会朝法线方向的某一边偏移，我们引入经典的洛伦兹力图像定量地解释了这一现象。通过分析狄拉克费米子的回旋轨道，我们解析推导了费米子透射穿过异质结的角度关系、全反射情况下的临界磁场。当电子能量接近势垒区的狄拉克点时，磁场诱导产生一个透射禁区。解析结果与非平衡格林函数方法计算得到的数值结果拟合得非常好。

另一有趣的狄拉克费米子体系是拓扑绝缘体。拓扑绝缘体表面态呈线性色散关系，受到时间反演对称性的保护[9-10]。同样的，拓扑绝缘体也具有Klein隧穿的典型特征，这一特性可用来设计新颖的拓扑绝缘体器件。我们采用散射矩阵方法，研究了含时电场作用下表面态的输运特性[11]。当电极无偏压时，垂直入射电子能完全透射穿过含时电场区，而有偏压时，Klein隧穿效应受到破坏，体系电导显著减小，自旋极化的左手螺旋特性也发生变化。含时电场对大角度透射曲线调节效果明显，产生震荡曲线。

1) M. I. Katsnelson, K. S. Novoselov, and A. K. Geim, Nature Physics 2, 620 (2006).

2) A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, Rev. Mod. Phys. 81, 109 (2009).

3) H. Y. Chen, V. Apalkov, and T. Chakraborty, Phys. Rev. Lett. 98, 186803 (2007).

4) P. Hewageegana and V. Apalkov, Phys. Rev. B 77, 245426 (2008).

5) A. D. Martino, L. DellAnna, and R. Egger, Phys. Rev. B 98, 066802 (2007).

6) M. Ramezani Masir, P. Vasilopoulos, A. Matulis, F. M. Peeters, Phys. Rev. B 77, 235443 (2008).

7) S. Moriyama, Y. Morita, E. Watanabe, and D. Tsuya, Appl. Phys. Lett. 104, 053108 (2014).

8) Yuan Li, Mansoor B. A. Jalil, and Guanghui Zhou, Appl. Phys. Lett. 105, 193108 (2014).

9) M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045-3067 (2010).

10) X. L. Qi and S. C. Zhang, Rev. Mod. Phys. 83, 1057-1110 (2011).

11) Yuan Li, Mansoor B. A. Jalil, S. G. Tan, W. Zhao, R. Bai and G. H. Zhou, Scientific Reports  4, 4624 (2014).

锰搀杂的稀磁半导体是一种前景看好的功能材料，它把半导体的特性和稳定的磁性有机地结合起来，在自旋电子学领域具有潜在的应用价值。在Mn基Ⅱ-Ⅵ稀磁半导体异质结构中,导带电子和Mn²⁺离子局域的3d电子通过sp-d交换发生相互作用。当沿着异质结构的生长方向施加一个外磁场时,异质结构中的顺磁层对于自旋向下的电子, 其行为等同于一个势阱,而对于自旋向上的电子来说则相当于一个势垒。由于势阱势垒对电子的散射存在着显著的差异,从而导致具有不同自旋指向的电子渡越稀磁半导体异质结构呈现出显著不同的依赖于自旋指向的效应。

超晶格结构由于层间的耦合作用，显现出许多不同于简单异质结构的特性。我们的研究表明Mn基周期Ⅱ-Ⅵ稀磁半导体/半导体超晶格结构可以在很宽广的能区对自旋向上或向下的电子进行过滤, 且自旋过滤带的宽度可由外磁场或超晶格的结构参数进行调节，这些性质表明该结构可以作为一种可调的自旋过滤器。如果周期超晶格结构中嵌入一层“杂质层”，则导致低能区透射概率的强烈衰减及依赖于电子自旋指向的单峰共振，理论上指出依赖于电子自旋指向的单峰共振可以用来设计单能量的自旋过滤器。

最近基于周期Ⅱ-Ⅵ稀磁半导体/非磁垒超晶格我们提出一种新颖的自旋过滤结构，研究表明与周期Ⅱ-Ⅵ稀磁半导体/半导体超晶格结构相比，周期Ⅱ-Ⅵ稀磁半导体/非磁垒超晶格可以在更宽广的入射能区获得100%自旋过滤效果，且自旋过滤带的宽度和位置可以由外磁场和超晶格的结构宽度进行有效地调控。这种优异的自旋过滤性质起源于非磁垒及由外磁场引起的依赖于自旋的巨Zeeman效应双重调制的结果。此外，研究同时表明外电场对超晶格结构中的自旋极化输运有很大影响。

 

参考文献:

[1] J.C. Egues, Phys. Rev. Lett. 80, 4578 (1998).

[2] J.C. Egues, C. Gould, G. Richter, and L.W. Molenkamp, Phys. Rev. B 64, 195319 (2001).

[3] Y. Guo, X.Y. Chen, F. Zhai, B.L. Gu, and Y. Kawazoe, Appl. Phys. Lett. 80, 4591 (2002).

[4] F. Zhai, Y. Guo, and B.L. Gu, J. Appl. Phys. 94, 5432 (2003).

[5] M. Büth, C. Gould, and L.W. Molenkamp, Phys. Rev. B 83, 155408 (2011).

[6] Y. Guo, F.R. Shen, and X.Y. Chen, Appl. Phys. Lett. 101, 012410 (2012).

[7] P.F. Yang, R. Zhu, and Y. Guo (submitted).

 

 

^*E-mail: guoy66@tsinghua.edu.cn

本课题得到了国家自然科学基金(课题号11174168)及973项目(课题号2011CB606405)的资助.

硅烯具有与石墨烯相似的二维原子晶格结构和电子能带结构，是近年来低维材料领域关注的热点之一，也是未来硅基电子学器件可能的构成材料。本课题组针对自由和在金属衬底上硅烯的生长机制、缺陷、能隙调控、氧化行为开展较为系统的第一性原理计算研究。研究了金属银衬底表面硅团簇结构随尺寸的演化，揭示了硅烯的初期成核生长机理，预言了硅烯与衬底之间重要的p-d杂化效应；给出了与实验STM相吻合的硅烯点缺陷形貌，揭示了点缺陷对硅烯电子结构的影响；考察了硅烯在不同惰性衬底表面的电子结构，发现衬底功函数对于硅烯电子结构的影响；提出了碱金属掺杂打开双层硅烯能隙的思路；研究了单层和多层硅烯的氧化行为和相应的电子态密度变化，解释了相关的实验。

The magnetism and electronic structures for carbon doped CdSe are investigated by using the full potential linearized augment plane wave method. Calculations show carbon substituting selenium could induce CdSe to be a diluted magnetic semiconductor. Single carbon dopant could induce 2.00μ_B magnetic moment. Electronic structures show the long-range ferromagnetic coupling mainly originates from the p-d exchange-like p-p coupling interaction. Positive chemical pair interactions indicate carbon dopants would form homogeneous distribution in CdSe host. Moderate formation energy (1.83eV) implies carbon doped CdSe can be realized experimentally.

锰搀杂的稀磁半导体是一种前景看好的功能材料，它把半导体的特性和稳定的磁性有机地结合起来，在自旋电子学领域具有潜在的应用价值。在Mn基Ⅱ-Ⅵ稀磁半导体异质结构中,导带电子和Mn²⁺离子局域的3d电子通过sp-d交换发生相互作用。当沿着异质结构的生长方向施加一个外磁场时,异质结构中的顺磁层对于自旋向下的电子, 其行为等同于一个势阱,而对于自旋向上的电子来说则相当于一个势垒。由于势阱势垒对电子的散射存在着显著的差异,从而导致具有不同自旋指向的电子渡越稀磁半导体异质结构呈现出显著不同的依赖于自旋指向的效应。

超晶格结构由于层间的耦合作用，显现出许多不同于简单异质结构的特性。我们的研究表明Mn基周期Ⅱ-Ⅵ稀磁半导体/半导体超晶格结构可以在很宽广的能区对自旋向上或向下的电子进行过滤, 且自旋过滤带的宽度可由外磁场或超晶格的结构参数进行调节，这些性质表明该结构可以作为一种可调的自旋过滤器。如果周期超晶格结构中嵌入一层“杂质层”，则导致低能区透射概率的强烈衰减及依赖于电子自旋指向的单峰共振，理论上指出依赖于电子自旋指向的单峰共振可以用来设计单能量的自旋过滤器。

最近基于周期Ⅱ-Ⅵ稀磁半导体/非磁垒超晶格我们提出一种新颖的自旋过滤结构，研究表明与周期Ⅱ-Ⅵ稀磁半导体/半导体超晶格结构相比，周期Ⅱ-Ⅵ稀磁半导体/非磁垒超晶格可以在更宽广的入射能区获得100%自旋过滤效果，且自旋过滤带的宽度和位置可以由外磁场和超晶格的结构宽度进行有效地调控。这种优异的自旋过滤性质起源于非磁垒及由外磁场引起的依赖于自旋的巨Zeeman效应双重调制的结果。此外，研究同时表明外电场对超晶格结构中的自旋极化输运有很大影响。

 

参考文献:

[1] J.C. Egues, Phys. Rev. Lett. 80, 4578 (1998).

[2] J.C. Egues, C. Gould, G. Richter, and L.W. Molenkamp, Phys. Rev. B 64, 195319 (2001).

[3] Y. Guo, X.Y. Chen, F. Zhai, B.L. Gu, and Y. Kawazoe, Appl. Phys. Lett. 80, 4591 (2002).

[4] F. Zhai, Y. Guo, and B.L. Gu, J. Appl. Phys. 94, 5432 (2003).

[5] M. Büth, C. Gould, and L.W. Molenkamp, Phys. Rev. B 83, 155408 (2011).

[6] Y. Guo, F.R. Shen, and X.Y. Chen, Appl. Phys. Lett. 101, 012410 (2012).

[7] P.F. Yang, R. Zhu, and Y. Guo (submitted).

We construct minimum effective models to investigate the pairing symmetry in the newly discovered quasi one-dimensional superconductor K₂Cr₃As₃. We show that a minimum three-band model based on the d_z² , d_xy and d_x²-y² orbitals of one Cr sublattice can capture the band structures near Fermi surfaces. In both weak and
strong coupling limits, the standard random phase approximation (RPA) and mean-field solutions consistently yield the triplet p_z-wave pairing as the leading pairing symmetry for physically realistic parameters. The triplet pairing is driven by the ferromagnetic fluctuations within the sublattice. The gap function of the pairing state possesses line gap nodes on the k_z = 0 plane on the Fermi surfaces. We also study the physical properties of this exotic triplet pairing state, which can be detected by different experiments.

We study the electronic structure of the quasi-one-dimensional material K_x Cr_3 As_3 using the first principles method. The K233 material is paramagnetic but close to magnetic instabilities. Its Fermi surface consists of two quasi-1D sheets and one 3D sheet. Electron susceptilibity suggests possible ferromagnetic spin fluctuation in the compound, which may lead to spin triplet pairing. The ground state of the K133 material is inter-layer antiferromagnetic (or A-type AFM) in first principles calculation. However, the 1D nature is greatly enhanced in the K133 material, whose Fermi surface involves only one-dimensional sheets. By fitting a twisted spin tube model, the intratube magnetic frustrations are found to be relaxed, leading to gapless spin excitations; while the intratube magnetic interactions are determined to be negligible. Therefore, the K133 material is a remarkable example of a nearly one-dimensional spin-chain. Upon increasing the intralayer exchange interactions, a frustration-induced  transition to disordered low block-spin state is expected.

BiFeO3是铁电性能非常突出的多铁材料，并且具有很高的反铁磁转变温度。美中不足的是其本征磁电耦合不强。本报告将设计一种基于BiFeO3的异质结体系来实现较强的电控磁效应。

We study the second-order Raman process of mono- and few-layer MoX2(X=S, Se, Te), by combining ab initio density functional perturbation calculations with experimental Raman spectroscopy using a number of excitation lasers. The calculated electronic band structure and the density of states show that the resonance Raman process occurs at the M point in the Brillouin zone, where a strong optical absorption occurs due to a logarithmic Van Hove singularity of the electronic density of states. The double resonance Raman process with intervalley electron-phonon coupling connects two of the three inequivalent M points in the Brillouin zone, giving rise to second-order Raman peaks due to the M-point phonons. The calculated vibrational frequencies of the second-order Raman spectra agree with the observed laser-energy-dependent Raman shifts in the experiment.

 

[1] H Guo, T Yang, M. Yamamoto et al., Phys. Rev. B 91:205415 (2015)

[2] H.-L. Liu, H. Guo, T. Yang, Z.Zhang et al., Phys.Chem.Chem.Phys. 17:14561 (2015)

[3] R. Saito, A. R. Nugraha, E. H. Hasdeo, S. Siregar, Huaihong Guo, Teng Yang, Physica Status Solidi B (2015).

 

 

石墨烯的成功，激发了探求其他的二维烯。硅烯由于能隙为零，并不能直接做高效的晶体管。我们介绍几种打开能隙的方法，以及相应的器件表现。继硅烯之后，磷烯最近又出场了。 我们系统研究了单层磷烯与金属电极的界面。发现界面总是出现肖特基势垒。实验上磷烯的亚阈值摇摆明显很大。我们模拟的磷烯器件。如果没有声子的影响， 亚阈值摇摆可以很小。但考虑声子，尤其是垂直平面的声子，亚阈值摇摆会增大许多。我们提出一个的方案，即用六角氮化硼夹住磷烯，这样可以有效压制垂直平面运动，从而得到高的器件表现。

 

   

References

1. 吕劲  等，Advanced Functional Materials 25, 68 (2015).
2. 吕劲  等，Scientific Reports 5, 9075 (2015).< > 等，
3. 吕劲 Nanoscale 6, 7609 (2014).
4. 吕劲 等，Nano Lett. 12, 113 (2012).
5. 吕劲 等，Scientific Reports  2：853 (2012).

    

The colossal magnetoresistance (CMR) effect known as a dramatic change of resistivity in magnetic field is a pretty useful property for application in magnetic storage techniques. The effect has been observed in materials like, hexaborides, Mn based pyrochlors, Cr based spinels and mixed-valent rare-earth manganites. Despite much efforts have been devoted, so far the general explanation for the CMR effect in various materials, however, has not yet reached [1]. In contrast to extensively studied sperovskite manganites [2], here we systematic investigate the electric transport and magnetization properties in spinel material HgCr2Se4 around metal-insulator transition (MIT). According to our critical exponent analysis, HgCr2Se4 turns out to be almost an ideal 3D Heisenberg type FM. In addition, without the mixied valent ions and Jahn-Teller effect [3], HgCr2Se4 can view as a textbook system to study the CMR effect. Around MIT, the deviation of reciprocal susceptibility to the Curie Wiess law implies that spins had correlated locally prior FMT, results the formation of short range FM orders. The resistivity presents an activation behavior at this region, as well as the change of lattice parameter suggests a consensus lattice distortion. These all coherently give strong evidences for the existence of Magnetic Polarons (MPs) around MIT. Besides, the magnetoresistivity being scaled with the magnetization further confirms the magnetic fluctuation nature around MIT that yields spin disorders. Our results highlight the critical role of MPs for CMR effect in HgCr2Se4, the origin of which has been elusive for past few decades. [1] A. P. Ramirez, J. Phys. Condens. Matter 9, 8171 (1997). [2] E. Dagotto, T. Hotta, and A. Moreo, Phys. Rep. 344, 1 (2001). [3] E.L. Nagaev, Physics Letters A. 239, 321-327 (1998).

    We study the temperature behaviors of the Hall coefficient, R_H , and longitudinal conductivity, σ, of two series of Ag_x(SnO₂)_1−x (x being the Ag volume fraction) granular films lying in the metallic regime. The first (second) series of films are ~500 (~9) nm thick and constitute a three- (two-) dimensional granular array. In the high-x regime with g_T » g_T^c (g_T being the dimensionless intergrain tunneling conductance, and g_T^c being the critical tunneling conductance at the percolation threshold), we observe a R_H ∝ lnT law from ~6 to 300K. This lnT behavior is independent of array dimensionality. We also observe a σ ∝ lnT law in the temperature range ~20–100 K. Below ~10 K, the temperature behavior of σ changes to a √T dependence in thick films, while it changes to a different lnT dependence in thin films. The overall R_H and σ characteristics can be explained by the electron-electron interaction (EEI) effects in the presence of granularity. As x is reduced and approaches the percolation threshold, we found that the lnT dependences of R_H and σ still hold for a wide temperature range. We propose an explanation for the long-standing puzzle of the σ ∝ lnT dependence, which has previously been frequently observed in composite systems near the quantum percolation threshold, as arising from the same EEI effect considered in the recent theory of granular metals.

The integer quantum Hall effect is a topological state of quantum matter in two dimensions, and has recently been observed in three-dimensional topological insulator thin films. Here we study the Landau levels and edge states of surface Dirac fermions in topological insulators under strong magnetic field. We examine the formation of the quantum plateaux of the Hall conductance and find two different patterns, in one pattern the filling number covers all integers while only odd integers in the other. We focus on the quantum plateau closest to zero energy and demonstrate the breakdown of the quantum spin Hall effect resulting from structure inversion asymmetry. The phase diagrams of the quantum Hall states are presented as functions of magnetic field, gate voltage and chemical potential. This work establishes an intuitive picture of the edge states to understand the integer quantum Hall effect for Dirac electrons in topological insulator thin films.

Multilayer silicene (MLS) draws intensive attentions, e.g., in chiral superconducting behavior, valley-polarized quantum Hall effect, and dramatically tunable electronic structure by varying stacking modes. Combining scanning tunneling microscopy and first principles investigations, we identify that the presumable van der Waals packed multilayer silicene sheets spontaneously transform into diamond-structure bulk Si film due to strong interlayer couplings. In contrast to drastic surface reconstruction on conventional Si(111), multilayer silicene prepared by bottom-up epitaxy on Ag(111) exhibits nearly ideal flat surface with only weak buckling. Without invoking Ag surfactants, √3×√3 honeycomb patterns emerge thanks to dynamic fluctuation of mirror-symmetric rhombic phases, similar to monolayer silicene [Chen et al., Phys. Rev. Lett. 110, 085504 (2013)]. The weak relaxation enables novel surface states with a Dirac linear dispersion. 

   基于密度泛函理论，我们研究了ScFeO₃的晶体结构，电子结构，和磁电性质。第一性原理计算结果表明：ScFeO₃的空间群为R3c，是具有菱面体畸变的钙钛矿结构。同时体系为反铁磁绝缘体，这与实验结果相一致[1]。基于贝利相位方法，我们进一步计算了ScFeO₃的自发电极化，发现其强度高达110.6 μC/cm²，远远超过了目前广泛应用的PZT材料，甚至超过了近年来研究热点材料BiFeO₃。我们发现如此大的自发电极化强度是和铁电相中较大的原子极化位移相一致的。而且进一步空间电荷分布计算表明，正是Sc 3d轨道和O 2p轨道的杂化导致了ScFeO₃中较大的铁电畸变。我们的研究不仅揭示了ScFeO₃材料的铁电起源，而且显示了其在铁电和多铁器件上的潜在应用 [2]。

 

[1] M. R. Li et al. J. Am. Chem. Soc. 134, 3737 (2012)

[2] S. C. Liu, T. Y. Cai, and S. Ju (submitted)

基于GGA+U方法，我们研究了氧空位对钙钛矿型过渡金属氧化物CaMnO3的晶体结构，电子结构和磁电性质的影响。我们发现，随着氧空位浓度的增加，体系由G类反铁磁序向铁磁序过渡，并伴随着绝缘体到金属态的转变。与之前锰氧化物中所讨论的电子掺杂导致的自陷型磁极化子相比[1]，由氧空位导致的局域态显示出更复杂的物理图像。除了传统束缚型极化子模型所描述的物理图像外，局域铁磁序所带来磁性势阱以及Mn⁴⁺离子上e_g轨道的占据对极化子的稳定也有着重要作用[2]。

[1] H. Meskine, T. Saha-Dasgupta, and S. Satpathy, Phys. Rev. Lett. 92, 056401 (2004)

[2] J. H. Dai, F. Hou, T. Y. Cai, and S. Ju (submitted)

Nanocrystalline Na_0.5Bi_0.5TiO₃ was prepared by sol- gel method. The nanocrystalline Na_0.5Bi_0.5TiO₃ plates present room temperature ferromagnetism. Vacuum annealing weakens ferromagnetism which disappears for the plate with subsequent long time and high temperature air- annealing, indicating that the observed ferromagnetism is connected with the cation vacancies. The LDA + U calculation results on the magnetism of Na0.5Bi0.5TiO3 (100) surface show that, Na vacancies can introduce a ferromagnetic at the surface of Na_0.5Bi_0.5TiO₃. The Na_0.5Bi_0.5TiO₃ plates annealed at 900°C for 1 hour show the coexistence of ferromagnetism and ferroelectricity at room temperature. The room- temperature magnetodielectric effect was observed in Na0.5Bi0.5TiO3 plates. In addition, the electric  field treatment leads to an enormous enhancement of saturation magnetization for Na_0.5Bi_0.5TiO₃ multiferroic plate, showing a strong magnetoelectric  coupling.

We investigate the effect of a vertical electric field on a Dirac semimetal thin film.  We show that through the interplay between the quantum confinement effect and the field-induced coupling between sub-bands, the

sub-band gap can be tuned and inverted, during which the system undergoes a topological phase transition between a trivial band insulator and a quantum spin Hall insulator. Consequently, one can electrically switch the

topological edge channels on and off, making the system a promising platform for constructing a topological field effect transistor.

arXiv:1503.08075

The Zn_0.32Co_0.68O_1−v/Pb hybrid junctions were prepared, where the concentrated magnetic semiconductor Zn_0.32Co_0.68O_1−v is in the region of variable range hopping transport instead of the ballistic or diffusive transport. The high differential conductance peak at gap voltage and two above-gap peaks were observed below the superconducting critical temperature. Moreover, both the zero bias conductance peak and the finite bias conductance peak were observed below the gap voltage. All these differential conductance peaks systematically evolve and finally disappear as the temperature or the magnetic field increases. These transport phenomena were explained by phase coherent Andreev reflection in the presence of strong disorder, magnetic impurity scattering, and spin polarization.

We demonstrate that it is possible to engineer Peierls-type quasi-1D systems on simple surfaces without introducing metallic deposits. We propose that rows of surface metal atoms on partially hydrogenated semiconducting wurzite (10-10) and zinc blende (110) surfaces may be viewed as 1D metallic structures that are exactly one atom across, highly uniform and having a very high L/w ratio. First principles DFT theory is used and the results show that the (1x1) surface is metallic and unstable, and a condensation at appropriate temperature to a more stable semiconductor phase/phases would occur. Detailed mapping of filled states near the Fermi level indicates that the energy gained is mainly from Fermi level electron redistribution as a gap opens due to the CDW. The idea of surface scaffolding is introduced. Through scaffolding, rows of surface metal atoms original to the compound are allowed constrained degrees of freedom in charge hoping and atomic distortions, for which other atoms in the solid support but do not participate in. The rows of surface metal atoms, constrained by the scaffold to exercise in simple up-down and/or sideways distortions and coupled to Fermi level nesting and CDW creation below the transition temperature, fulfill the paradigm envisioned in Peierls’ original description.

In this work, the diffusion of oxygen vacancy from CeO₂ (111) subsurface to surface has been studied by density function al calculations. On the clean CeO₂ (111) surface, the diffusion barrier of oxygen vacancy from subsurface to surface is 0.34 eV, while O₂ adsorbs on surface, the diffusion barrier disappears. For CeO₂ (111) surface adsorbed by H₂O, two different interactive processes of H₂O and subsurface oxygen vacancy has been proposed. In the process of H₂O dissociation following in oxygen vacancy diffusion, two barriers of 0.22 eV and 0.15 eV are exist in the whole pathway, while for the process of H₂O dissociation following by oxygen vacancy diffusion, the barriers of whole pathway are only 0.09 eV and 0.06 eV. Therefore, the second pathway most probably exist in the interaction between H₂O and CeO₂ (111) subsurface oxygen vacancy. In addition, the dynamical behaviors of O₂ filled in CeO₂ (111) surface oxygen vacancy have also been studied. Below 200 K, one O atom of O₂ always rolls round the other. When temperature increases to 400 K, their sites will exchange.

Using first-principles calculations, we present a theoretical study of the structural, electronic and magnetic properties of substitutional 3d transition metal (TM) impurities in two-dimensional black and blue phosphorenes. We pay special attention to the magnetic properties of these substitutional impurities, which can be understood in terms of a simple model based on the Hund's rule. For TM-doped black phosphorene, the calculated band structures of substitutional Ti, V, Cr, Mn, Fe and Ni impurities show dilute magnetic semiconductor (DMS) properties while those of substitutional Sc and Co impurities show nonmagnetic property. We find that such TM-doped black phosphorenes with Ti, V, Cr, Mn, Fe and Ni impurities show dilute magnetic semiconductor (DMS) properties while those with Sc and Co impurities show nonmagnetic properties. On the other hand, the TM-doped blue phosphorenes with V, Cr, Mn and Fe impurities show DMS properties, those with Ti and Ni impurities show half-metal properties, whereas those with Sc and V impurities show nonmagnetic properties. We identify two different regimes associated with the occupation of differently hybridized TM-phosphorous electronic levels: (i) bonding states are completely empty or filled for Sc- and Co-doped black/blue phosphorenes, leading to non-magnetic; (ii) non-bonding d states are partially occupied for Ti-, V-, Cr-, Mn-, Fe- and Ni-doped black/blue phosphorenes, giving rise to large and localized spin moments. These results provide a new route for the potential implication of dilute magnetic semiconductor and half-metal in spintronic devices based onblack/blue phosphorene.

We investigate the selection rule and optical spectrum for armchair-edge silicene nanoribbons with N atomic chains (N-ASiNRs) under the irradiation of an external electromagnetic field at low temperatures. By using the method of the dipole-transition theorem for semiconductors, both the semiconducting and metallic ASiNRs have been demonstrated to own a broad optical conductivity and dielectric function from infrared to ultraviolet. The optical spectra for semiconducting 9- and 10-ASiNRs are observed to the transitions between the conduction and valence subbands with the same and/or different indices, while those for metallic 11-ASiNR may be only originated from the transitions between the conduction and valence subbands with different indices. The obtained results are believed to be of importance in exploring the new effects and optoelectronic applications of the silicene-based devices.

摘要：我们采用单源气相沉积的方法制备了大面积的少层二硫化钼纳米薄膜。用SEM，AFM以及拉曼对其结构和形貌进行表征，结果证明制备所得的二硫化钼纳米薄膜是少层的，连续的以及高质量的。同时，我们对所得二硫化钼纳米薄膜分别在不同的偏压下进行了光响应的测试，结果表明，光照时，电流明显增加，最高时达到了21.2 mA，随着施加的偏压增加，光电流随之增大，表明电压辅助的电荷分离机制；此外，光响应测试的结果发现施加电压为0.5 V，1 V，或者1.5 V时，测试得的光响应反应时间快，稳定，而且可循环，在每个偏压下的光响应衰减时间及增长时间都很接近，表明了制备所得的二硫化钼纳米片有良好的光响应性能。

 

关键词：二硫化钼  光响应 气相沉积 电荷分离

 

The electronic structure of the antiferromagnetic heavy fermion compound CeIn₃(T_N=10K) in the paramagnetic state has been studied by angle-resolved photoelectron spectroscopy(ARPES) using soft x-rays (hν = 535–900 eV). Heavy quasiparticle band with hybridized Ce 4f and In 5p signals were observed in the ARPES spectra and form Fermi surfaces, suggesting that Kondo lattice system also havs itinerant components. The hybridization effect also shows obvious spatial distribution from our ARPES data, giving some possible electronic explanation  of the dual characterof the  f electrons.

We present first-principles density-functional calculations for the structural, electronic, and mag- netic properties of substitutional 3d transition metal (TM) impurities in two-dimensional black and blue phosphorenes. We find that the magnetic properties of such substitutional impurities can be understood in terms of a simple model based on the Hund’s rule. The TM-doped black phosphorenes with Ti, V, Cr, Mn, Fe and Ni impurities show dilute magnetic semiconductor (DMS) properties while those with Sc and Co impurities show nonmagnetic properties. On the other hand, the TM- doped blue phosphorenes with V, Cr, Mn and Fe impurities show DMS properties, those with Ti and Ni impurities show half-metal properties, whereas Sc and Co doped systems show nonmagnetic properties. We identify two different regimes depending on the occupation of the hybridized elec- tronic states of TM and phosphorous atoms: (i) bonding states are completely empty or filled for Sc- and Co-doped black and blue phosphorenes, leading to non-magnetic; (ii) non-bonding d states are partially occupied for Ti-, V-, Cr-, Mn-, Fe- and Ni-doped black and blue phosphorenes, giving rise to large and localized spin moments. These results provide a new route for the potential applications of dilute magnetic semiconductor and half-metal in spintronic devices by employing black and blue phosphorenes.

previous studies found multiple signatures of an imminent magnetic order in the compound YeFe2Al10, however were unable to observe the transition to a magnetically ordered state in temperatures below 100 mK. To further investigate the magnetic instability, we performed measurements of magnetic susceptibility and muon spin relaxation on single crystalline YFe2Al10.

通过化学掺杂或者施加高压等调控手段将长程磁有序抑制掉可以实现磁性量子临界点，在其附近往往伴随出现诸如非费米液体行为或者非常规超导电性等反常的物理现象。相对于化学掺杂，高压调控具有不引入晶格无序和实现精细调控等优点。本报告中，我将详细介绍我们最近利用高压调控手段将螺旋磁体CrAs和MnP的长程磁有序压制掉而在其磁性量子临界点附近发现超导电性的最新进展[1, 2]。常压下，CrAs在T_N =265 K发生一级反铁磁相变，低温形成双螺旋磁有序；随着施加高压，T_N迅速向低温移动，大约在P_c ~1 GPa完全消失，在此压强附近我们观察到T_sc ~ 2K的体超导。而MnP在常压下经历了两个磁相变，首先在T_C = 290 K附近形成长程铁磁序，然后在T_s = 50K附近转变为双螺旋反铁磁序。随着施加高压，T_s迅速降低，大约在1 GPa完全消失；T_C也逐渐降低，在3-4 GPa从铁磁序转变为反铁磁序，并随着压强增加继续降低，最终在P_c ~ 8 GPa完全被压制掉；在P_c附近很窄的压强范围内我们观察到T_SC ~  K的超导电性。通过详细的高压下电阻率、磁化率和中子散射等测试，我们绘制了CrAs和MnP的温度-压强相图，得到与一些典型的非常规超导体系，诸如重费米子和铁基等，非常类似的相图，而且发现在临界压强P_c附近存在rµT^1.5非费米液体行为和电子有效质量的显著提高等现象，表明与反铁磁量子临界点相关的临界自旋涨落与超导机制具有重要关联。这是首次在Cr和Mn基化合物中观察到超导电性，为探索新的非常规超导体打开了一扇大门。

 

致谢：本工作是与中科院物理所的雒建林研究员、吴伟、孙建平、林富锟，日本东京大学的Yoshiya Uwatoko教授和Kazuyuki Matsubayashi博士，美国橡树岭国家实验室的Masaaki Matsuda博士等共同完成的。另外，感谢国家自然基金委、科技部和中科院先导B项目的支持。

 

参考文献：

1. W. Wu, J.-G. Cheng, K. Matsubayashi, P. P. Kong, F. K. Lin, C. Q. Jin, N. L. Wang, Y. Uwatoko, and J. L. Luo; “Superconductivity in the vicinity of antiferromagnetic order in CrAs”; Nature Communications (2014) 5, 5508。

J.-G. Cheng, K. Matsubayashi, W. Wu, J. P. Sun, F. K. Lin, J. L. Luo, and Y. Uwatoko; “Pressure induced superconductivity on the border of magnetic order in MnP”; Physical Review Letters (2015) 114, 117001.

外尔半金属是最近实验证实的一种全新的拓扑量子材料。它的体电子态中存在手性相反的半狄拉克型外尔节点，而表面态则为连接手性相反的外尔点的分离的费米弧。最近的角分辨光电子谱实验在对非中心对称的半金属材料TaAs以及其同结构的化合物上首次发现了外尔电子存在的证据。我们对TaAs等化合物的电输运表征表明它们是具有超高迁移率和极小电子浓度的半金属。极强的SdH量子振荡以及巨大的线性磁阻表明这种材料具有很强的背向散射保护机制。同时我们还观测到由手性反常导致的纵向负磁阻效应。

Quantum Transport in Topological Semimetals: Weak anti-localization and High-field Negative Magnetoconductivity

 

卢海舟

Department of Physics, South University of Science and Technology, Shenzhen 518055, China

luhz@sustc.edu.cn

 

Topological semimetal is a three-dimensional topological state of matter, in which the conduction and valence energy bands touch at a finite number of Weyl nodes. The Weyl nodes always appear in pairs, in each pair the quasiparticles carry opposite chirality and linear dispersion, much like 3D analogue of graphene. Topological semimetals are three-dimensional topological states of matter, in a sense that they host paired monopole and antimonopole of Berry curvature in momentum space.

Weyl fermions are expected to be immune from localization. In a weak external magnetic field, a negative magnetoconductivity is found to be proportional to the square root of magnetic field at low temperatures, giving the signature of the weak anti-localization. However, we demonstrate that the weak anti-localization behavior of Weyl semimetals can be sabotaged by electron-electron interaction and inter-valley effects. A localization tendency is therefore illustrated in disordered and interacting Weyl semimetals.

A positive magnetoconductivity in strong and parallel fields has long been anticipated as one the signatures of the chiral anomaly of topological semimetals. However, almost all experiments on Weyl and Dirac topological semimetals show a negative magnetoconductivity in high fields. We study the magnetoconductivity of topological semimetals, in the presence of delta, screened Coulomb, and Gaussian potentials. In a strong parallel magnetic field, we identify various scenarios in which the high-field magnetoconductivity may be negative. Our findings show that the high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly and will be helpful for interpreting the inconsistency in the recent experiments and earlier theories.

 

References:

[1] Hai-Zhou Lu and Shun-Qing Shen, arXiv:1411.2686.

[2] Hai-Zhou Lu, Song-Bo Zhang, and Shun-Qing Shen, arXiv:1503.04394.

报告的第一部分，我们将介绍FeSe基超导体Fe_1+yTe_1-xSe_x体系中的非弹性中子散射的研究结果。结合输运测量，着重展示该体系中低能磁激发与超导随温度以及掺杂的演变过程；第二部分，我们将介绍另一类FeSe基超导体K_2-xFe_ySe₂体系中中子散射的研究结果。我们将报道近期我们在该体系中观察到的静态磁有序、以及低能磁激发与该样品中超导相的耦合。

 

Prediction of Topological Materials from First-priniciples Calculation

Hongming Weng,^1,2

 

¹ Institute of Physics, Chinese Academy of Sciences, Beijing, China

² Collaborative Innovation Center of Quantum Matter, Beijing, China

 

Email: hmweng@iphy.ac.cn

 

In this talk, I will introduce our recent works on predction of topological materials, which possess nontrivial band topology, by using first-principles calculations. Firstly, I will introduce the theory of band topology and the Wilson loop method used for calcuating topological invariants in practice.[1] Secondly, I will show some examples, including the two-dimensional (2D) large-band-gap topological insulators ZrTe5/HfTe5[2] and the transition metal dichalcogenide (TMD) haeckelites with square-octagonal lattice.[3] Thirdly, topological semimetals, the extension of band topology to metal, are discussed and three members, the topologicla Dirac semimetal,[4] Weyl semimetal [5] and Node-Line semimetal,[6] are demonstrated.

 

[1]    Hongming Weng,^† Xi Dai and Zhong Fang, MRS Bulletin 39, 849 (2014)

[2]    Hongming Weng, Xi Dai and Zhong Fang, Phys. Rev. X 4, 011002 (2014)

[3]    S. M. Nie, Zhida Song, Hongming Weng,^† and Zhong Fang,^* Phys. Rev. B 91, 235434 (2015)

[4]    Z. Wang, Y. Sun, X. Chen, C. Franchini, G. Xu, Hongming Weng,^† Xi Dai and Zhong Fang,^* Phys. Rev. B 85, 195320 (2012); Z. Wang, Hongming Weng,^* Q. Wu, Xi Dai and Zhong Fang,^† Phys. Rev. B 88, 125427 (2013)

[5]    G. Xu, Hongming Weng, Z. Wang, Xi Dain and Zhong Fang,^* Phys. Rev. Lett. 107, 186806 (2011); Hongming Weng,^* C. Fang, Zhong Fang, B. A. Bernevig, and Xi Dai, Phys. Rev. X 5, 011029 (2015)

[6]    Hongming Weng^*, Y. Liang, Q. Xu, R. Yu, Zhong Fang, Xi Dai and Y. Kawazoe, arXiv:1411.2175; R. Yu, Hongming Weng^*, Zhong Fang, Xi Dai, and X. Hu, (arXiv:1504.04577) Phys. Rev. Lett. (accepted)

 

拓扑电子态是电子能带拓扑性质导致的一类新奇量子态. 它可以分成拓扑绝缘态和最近几年新发现的拓扑半金属态. 这两类拓扑态在凝聚态物理和材料科学中都受到广泛的关注.  拓扑半金属的费米面上只存在能带交叉点, 这些能带交叉点是晶格动量空间中的磁单极子. 根据磁单极子在晶格动量空间中的分布,拓扑半金属可以分为 Dirac 半金属, Weyl 半金属和 nodal-line 半金属这三类. 前两种拓扑态, 磁单极子在动量空间是孤立的点. 而在 nodal-line 半金属中, 磁单极子在动量空间中形成了连续的闭合曲线. 到目前为止, 前两种拓扑金属态的材料系统已经有理论预言并获得了实验证实. 而拓扑 nodal-line 半金属目前还没有被实验所观测到. Nodal-line 半金属非平庸的拓扑性质和特殊的能带交叉点构型, 使得在这类材料的表面上可以存在无色散的二维平带. 这种二维平带对研究电子间的关联作用, 分数拓扑绝缘体和高温超导都有非常重要的作用. 因此,寻找到能够实现这种新奇量子态的真实材料系统具有很重要的意义. 

本报告将介绍我们近期关于 nodal-line 半金属的理论研究工作. 通过第一性原理计算和模型分析, 我们预言了三维的全碳 Mackay-Terrones 晶体和反钙钛矿结构的 Cu₃PdN 晶体能够实现拓扑 nodal-line 半金属态. 这些发现, 为进一步研究 nodal-line 半金属的新奇物理性质提供了材料基础.

FeSe bulk superconductors, FeSe thin films, and other FeSe-based related superconductors have attracted much attention in the study of the iron-based superconductors [1]. The possible high Tc superconductivity and distinct electronic structure of these systems provide critical information in understanding the superconductivity mechanism of the iron-based superconductors. In this talk, I will present our angle-resolved photoemission study on the electronic structure and superconductivity of the A1-xFe2-ySe2 superconductor[2], FeSe/SrTiO3 thin films[3-6] and newly-discovered (LiFe)OHFeSe superconductor[7].

 

Ref：

[1]. X Liu et al 2015 J. Phys.: Condens. Matter 27 183201

[2]. D X Mou et al 2011 Front. Phys. 6 410

[3]. D F Liu et al 2012 Nat. Commun. 3 931

[4]. S L He et al 2013 Nat. Mater. 12 605

[5]. J F He et al 2014 Proc. Natl Acad. Sci. 111 18501

[6]. X Liu et al 2014 Nat. Commun. 5 5047

[7]. L Zhao et al 2015 Arixv: 1505.06361

FeSe bulk superconductors, FeSe thin films, and other FeSe-based related superconductors have attracted much attention in the study of the iron-based superconductors [1]. The possible high Tc superconductivity and distinct electronic structure of these systems provide critical information in understanding the superconductivity mechanism of the iron-based superconductors. In this talk, I will present our angle-resolved photoemission study on the electronic structure and superconductivity of the A1-xFe2-ySe2 superconductor[2], FeSe/SrTiO3 thin films[3-6] and newly-discovered (LiFe)OHFeSe superconductor[7].

 

Ref：

[1]. X Liu et al 2015 J. Phys.: Condens. Matter 27 183201

[2]. D X Mou et al 2011 Front. Phys. 6 410

[3]. D F Liu et al 2012 Nat. Commun. 3 931

[4]. S L He et al 2013 Nat. Mater. 12 605

[5]. J F He et al 2014 Proc. Natl Acad. Sci. 111 18501

[6]. X Liu et al 2014 Nat. Commun. 5 5047

[7]. L Zhao et al 2015 Arixv: 1505.06361

Single-layer FeSe film on SrTiO₃ (001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO₃(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle interference (QPI) patterns reveal scatterings between and within the electron pockets, and put constraints on possible pairing symmetries. By introducing impurity atoms onto the sample, we show that the magnetic impurities (Cr, Mn) can locally suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K) cannot. Our results indicate that single-layer FeSe/ SrTiO₃ has a plain s-wave paring symmetry whose order parameter has the same phase on all Fermi surface sections.

Helicity protected ultrahigh mobility Weyl fermions in NbP

Yi Zheng¹, Zhen Wang^1,2, Xiaojun Yang¹ , and Zhu-An Xu¹

¹Department of Physics, Zhejiang University, Hangzhou 310027, P. R. China

^*Correspondence to: phyzhengyi@zju.edu.cn; zhuan@zju.edu.cn

 

Non-centrosymmetric transition metal monopnictides, including TaAs, TaP, NbAs and NbP, have been very recently proposed to be Weyl semimetals (WSMs), in which the low-energy quasiparticle excitations are the condensed-matter-physics realization of the long-sought-out Weyl fermions, which previously only appear in theoretical high-energy physics. The first fundamental question in this emergent field is that which compound is the ideal platform for exploring various fascinating quantum phenomena and novel device concepts. Although chiral WSM states have been observed in TaAs and NbAs, there is no conclusive evidence on the existence of Weyl fermions in NbP. Here, we use angle-dependent quantum oscillations to reveal that NbP has four pairs of unusually large Weyl fermion pockets in the k_z=0 plane near the high symmetry points Σ, dominating over the coexisting massive hole pockets and the previous reported WSM pockets in the k_z=1.18π/c plane. Such dominant WSM pockets are highly anisotropic in k-space and approaching the parabolic band top along the internode direction. The corresponding Fermi surface is consisting of helical Weyl fermions with unprecedented mobility of 1×10⁷ cm²V^-1s^-1 at 1.5 K, well protected from defect backscattering by real spin conservation associated to the chiral Weyl nodes. Inter-pocket pumping of Weyl fermions with opposite helicity becomes feasible when the magnetic field and electric field are applied in parallel, manifested as robust chiral anomaly induced negative MR. Surprisingly, we have observed a strong offset to the π Berry’s phase, which may be correlated to the non-negligible mass of Weyl fermions.

References

[1] H. M. Wen, et al, Phys. Rev. X 5, 011029 (2015)

[2] Z. Wang, Y. Zheng, Z. Shen, Y. Zhou, X. Yang, Y. Li, C. Feng, and Z. A. Xu, arXiv:1506.00924.

Sr₂IrO₄ was  predicted  to  be  a  high  temperature  superconductor  upon electron  doping  since  it  highly  resembles  the  cuprates  in  crystal  structure, electronic structure and magnetic coupling constants. Here we report a scanning tunneling  microscopy/spectroscopy  (STM/STS)  study  of  Sr₂IrO₄ with  surface electron  doping  by  depositing  potassium  atoms.  At  the  0.5-0.7  monolayer  K  coverage,  we  observed  a  sharp,  V-shaped  gap  with  about  95%  loss  of density of state (DOS) at E_F and visible coherence peaks. The gap magnitude is 25-30 meV for 0.5-0.6 ML K coverage and it closes around 50 K. These behaviors exhibit  clear  signature  of  superconductivity.  Furthermore,  we  found  that  with increased electron doping, the system gradually evolves from aninsulating state to  a  normal  metallic  state,  via  a  pseudogap-like  state  and  possible superconducting  state.  Our  data  suggest  possible  high  temperature superconductivity in electron doped Sr₂IrO₄, and its remarkable analogy to the cuprates.

    We report the quantum transport properties of Cd₃As₂ single crystals in a magnetic field. A large linear magnetoresistance is observed near room temperature. With decreasing temperature, the Shubnikov-de Haas oscillations appear in both the longitudinal resistance R_xx and the transverse Hall resistance R_xy. From the strong oscillatory component ΔR_xx, a linear dependence of the Landau index n on 1/B is obtained, and it gives an n-axis intercept between 1/2 and 5/8. This clearly reveals a nontrivial π Berry’s phase, which is a distinguished feature of Dirac fermions. Our quantum transport results provide bulk evidence for the existence of a three-dimensional Dirac semimetal phase in Cd₃As₂ [1]. Interestingly, we observe superconductivity in Cd₃As₂ with Tc ~ 4 K under high pressure [2]. It is possible that Cd₃As₂ under pressure is a topological superconductor.

    We will also present the high-pressure study of the Weyl semimetal NbAs [3]. The crystal structure remains stable up to 26 GPa according to the powder XRD data. The resistance of NbAs single crystal increases monotonically with pressure at low temperature. Up to 20 GPa, no superconducting transition is observed down to 0.3 K. These results show that the Weyl semimetal phase is robust in NbAs, and applying pressure is not a good way to get a topological superconductor from a Weyl semimetal.

 

[1] L. P. He, X. C. Hong, J. K. Dong, J. Pan, Z. Zhang, J. Zhang, and S. Y. Li, “Quantum transport evidence for the three-dimensional Dirac semimetal phase in Cd₃As₂”, Phys. Rev. Lett. 113, 246402 (2014). Editors’ Suggestion.

[2] L. P. He, Y. T. Jia, S. J. Zhang, X. C. Hong, C. Q. Jin, and S. Y. Li, “Pressure-induced superconductivity in the three-dimensional Dirac semimetal Cd₃As₂”, arXiv:1502.02509.

[3] J. Zhang, F. L. Liu, J. K. Dong, Y. Xu, N. N. Li, W. G. Yang, and S. Y. Li, “High-pressure study of the Weyl semimetal NbAs”, arXiv:1506.00374.

    The discovery of materials with novel properties is one of the most fascinating aspects of physics, and such findings have always played important roles in the development of science and human life. Two recent examples are graphene and topological insulators. Interestingly, both materials possess 2D Dirac fermions; and topological insulators further show distinct topology in their electronic band structures. With the swift development in both fields, two questions have been naturally raised:

i). Does there exist a 3D counterpart of graphene, or a “3D graphene”?

ii). Besides topological insulators, can one find other materials that have unusual topology in their electronic structures?

    Remarkably, the answer to both questions can lie on a same type of novel quantum matter – the topological semi-metals - which not only processes 3D Dirac or Weyl fermions in the bulk (in contrast to the 2D Dirac fermions in graphene and topological insulators), but also shows unique surface states result from its non-trivial topology of the bulk electronic structure.

    In this talk, I will show that by using advanced photoemission spectroscopy with high energy, momentum, and time resolution, we were able to directly visualize the non-trivial electronic structures and unusual dynamics in topological insulators and topological semi-metals recently discovered. Finally, I will briefly discuss the application potentials of these unusual materials.

Degenerate groundstates and spontaneous symmetry breakings in quantum phase transitions: spin-1/2 plaquette chain systems Sam Young Cho (Chongqing University) Spontaneous symmetry breaking of the Landau theory provides the key concept to understand quantum phase transitions. It stats normally that groundstates have a lower symmetry than considering Hamiltonian for a given system parameter. However, groundstates can have more symmetry than Hamiltonian for a given parameter. For instance, the dimerized states as a groundstate can have a U(1) symmetry in the frustrated antiferromagnetic Heisenberg chain, so called, J1-J2 model. Such a situation is called emergent symmetry. However, the U(1) symmetry of the dimerized states has nothing to do with the spontaneous symmetry breaking, i.e., one-site translational symmetry breaking that characterizes the dimer phase in the frustrated antiferromagnetic Heisenberg chain. In this talk, an extended scenario of spontaneous symmetry breakings will be introduced. Based on groundstates in spin-1/2 plaquette chain systems, which can be calculated by using the infinite matrix product state (iMPS) representation with the infinite time evolving decimation block (iTEBD) method, a role of emergent symmetry for the groundstates will be discussed. A covering symmetry group over all emergent symmetries responsible for degenerate groundstates in the plaquette chain system is found to correspond to a largest common symmetry group of constituent Hamiltonians describing the plaquette system. Consequently, this result suggests that, as a guiding symmetry principle in quantum phase transitions, degenerate groundstates are induced by a spontaneous breaking of symmetries belonging to a largest common symmetry group of constituent Hamiltonians describing a given system, but each of them can have more symmetries than the largest common symmetry.

Novel properties of 5d transition metal oxides Xiangang Wan National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China In 5d transition metal oxides, novel properties arise from the interplay of electron correlations and spin-orbit interactions. In this talk, I will focus on describing the topological Weyl-Semimetal in pyrochlore iridates, the Axion insulatior in spinel osmates, the Slater insulator in perovskite osmates. I will also discuss the anisotropic unscreened Coulomb interaction in ferroelectric metal LiOsO3.

The amplitude and symmetry of the superconducting order parameter are intimately related to the pairing mechanism leading to unconventional high-temperature superconductivity in the Fe-based superconductors. Angle-resolved photoemission spectroscopy (ARPES) provides a direct characterization of the superconducting gap in the momentum space. Here I present ARPES results on ferropnictide and ferrochalcogenide superconductors suggesting that the pairing mechanism is not driven by the Fermi surface, but is rather consistent with short-range interactions. In addition, I discuss the role of the electronic correlations in these materials.

Understanding exotic, non s--wave--like states of Cooper pairs is importantand may lead to new superconductors with higher critical temperatures and novelproperties. Their existence is known to be possible but has always been thoughtto be associated with non--traditional mechanisms of superconductivity whereelectronic correlations play an important role. Here we use a first principleslinear response calculation to show that in doped Bi2Se3 anunconventional p--wave--like state can be favored via a conventionalphonon--mediated mechanism, as driven by an unusual, almost singular behaviorof the electron--phonon interaction at long wavelengths. This may provide a newplatform for our understanding superconductivity phenomena in doped bandinsulators.

Pressure-induced insulating state and superconductivity in bismuth tellurohalides

 

Xiao-Jia Chen

Center for High Pressure Science and Technology Advanced Research, Shanghai 201203, China

 

 

Searching for the topological superconductivity is being driven by the exploration of a novel matter and the potential applications in topological quantum computation. Layered non-centrosymmetric bismuth tellurohalides with large Rashba-type splitting in the bulk bands are being examined as the inversion asymmetric topological insulators. Pressure has been proposed to be essential for inducing and tuning topological order in these systems. Here we show that applying pressure indeed induces the theoretically predicted insulating state in Rashba semiconductor BiTeX (X=Cl, Br, I). Superconductivity is then followed with the insulating normal state. Upon heavy compression, another different superconducting phase is found with the metallic normal state. These findings not only enrich the superconducting family from topological insulators but also pave the road on the search of topological superconductivity in bismuth tellurohalides.

 

This work is in collaboration with Jian-Jun Ying, Fei Chen, Xian-Hui Chen, Viktor V. Struzhkin, Alexander F. Goncharov, Ho-Kwang Mao, and Alexander G. Gavriliuk.

We study an inhomogeneous three-orbital Hubbard model for the Cu substituted iron pnictides using a multiorbital real-space Green's function method combined with the density functional calculation. We find that the onsite interactions of the Cu ions is the principal determinant of whether an electron dopant or a hole dopant is caused by the Cu substitution, which can explain why the effects of Cu substitution on the carrier density are entirely different in NaFe_1−xCu_xAs and Ba(Fe_1−xCu_x)₂As₂. We also find that the effect of doping-induced disorder is crucial in the Cusubstituted iron pnictides, so is the Mott mechanism for localization, and their cooperative effect introduces an orbital-selective insulating phase.


 

By intercalation of alkaline-earth metal Sr in Bi2Se3, superconductivity with large shielding volume fraction (91.5% at 0.5 K) has been achieved in Sr0.065Bi2Se3. The analysis of the surface Shubnikov-de Hass oscillations confirms the 1/2-shift expected from a Dirac spectrum, giving transport evidence of the existence of surface states. Importantly, the SrxBi2Se3 superconductor is stable under air and exhibits very low electron carrier density, making the SrxBi2Se3 compound an ideal material base for investigating topological superconductivity.

By intercalation of alkaline-earth metal Sr in Bi2Se3, superconductivity with large shielding volume fraction (91.5% at 0.5 K) has been achieved in Sr0.065Bi2Se3. The analysis of the surface Shubnikov-de Hass oscillations confirms the 1/2-shift expected from a Dirac spectrum, giving transport evidence of the existence of surface states. Importantly, the SrxBi2Se3 superconductor is stable under air and exhibits very low electron carrier density, making the SrxBi2Se3 compound an ideal material base for investigating topological superconductivity.

CeRhIn₅是一个经典反铁磁近藤晶格材料。在外加压力或磁场下，CeRhIn₅表现出丰富的物理性质，如反铁磁态，超导态，非费米液体态等。通过测量强磁场下的交流比热、dHvA效应以及Hall效应，我们首次给出了重费米子材料CeRhIn₅在磁场沿样品的a-和c-轴时的磁场-温度相图，并在B_c0(0)=50T附近观测到了一个较为各向同性的反铁磁量子临界点。此外，我们在反铁磁态内(B^*≈30T)发现磁场诱导的费米面重构现象，该重构现象表现在dHvA频率和霍尔系数的突变，电子有效质量的增加以及熵的突然减小。该磁场诱导的费米面重构可能来源于Ce的4f电子通过近藤效应而产生的局域-巡游转变，这说明CeRhIn₅存在一个磁场诱导的三维巡游量子临界点(与压力诱导的局域量子临界点截然不同)。这表明，在同一材料中，不同的调控参量可诱导出不同类型的量子临界点，而费米面是表征不同类型量子临界点的一个重要物理参量。这些实验现象的发现将对建立统一的量子相变理论和相图起到极大的促进作用。

Evidence for nodal superconductivity in quasi-one-dimensional K₂Cr₃As₃

G. M. Pang¹, M. Smidman¹, W. B. Jiang¹, J. K. Bao¹, Z. F. Weng¹,

Y. F. Wang¹, L. Jiao¹, J. L. Zhang¹, G. H. Cao¹, and H. Q. Yuan^1,*

¹Department of Physics and Center for Correlated Matter, Zhejiang University, Hangzhou, 310027, China

 

The newly discovered quasi-one-dimensional superconductor K₂Cr₃As₃ has attracted considerable interest [1]. Its crystal structure consists of double-walled tubes of [Cr₃As₃]^2- that run along the c axis. Electronic structure calculations for K₂Cr₃As₃ reveal that three bands cross the Fermi level, two of which are one-dimensional and one is three-dimensional [2]. For a transition temperature of T_c≈6.1K, this compound shows a remarkably high upper critical field (B_c2(0) ≈31.2T), far exceeding the Pauli limiting value [1]. In this presentation, we report measurements of the London penetration depth delta lambda(T) and superfluid density rho_s(T) of K₂Cr₃As₃ to probe its superconducting gap structure [3]. Linear behavior of delta lambda(T) is observed for T<<Tc, instead of the exponential behavior of conventional superconductors, indicating that there are line nodes in the superconducting gap. Possible pairing states will be discussed by comparing different fits of rho_s(T). Our results provide strong evidence for unconventional behavior and may provide key information for identifying the pairing state of this novel superconductor.

 

References

[1]J. K. Bao et al., Superconductivity in quasi-one-dimensional K₂Cr₃As₃ with significant electron correlations. Phys.Rev.X.5, 011013.

[2]H. Jiang, G. H. Cao, and C. Cao, Electronic structureof quasi-one-dimensional superconductor K₂Cr₃As₃ from first-principles calculations. arXiv:1412.1309.

[3]G. M. Pang et al., Evidence for nodal superconductivity in quasi-one-dimensional K₂Cr₃As₃. arXiv:1501.01880.

We report measurements of the physical properties and electronic structure of the hexagonal compounds Yb2Ni12Pn7 (Pn = P, As) by measuring the electrical resistivity, magnetization, specific heat, partial fluorescence yield x-ray absorption spectroscopy (PFY-XAS), and resonant x-ray emis-sion spectroscopy (RXES). These demonstrate a crossover upon reducing the unit cell volume, from a mixed valence state in Yb2Ni12As7 to a heavy-fermion paramagnetic state in Yb2Ni12P7, where the Yb is nearly trivalent. Application of pressure to Yb2Ni12P7 suppresses TFL, the temperature below which Fermi liquid behavior is recovered, suggesting the presence of a quantum critical point (QCP) at around 2.8 GPa. However, while there is little change in the Yb valence of Yb2Ni12P7 up to 30 GPa, there is a strong increase for Yb2Ni12As7 under pressure, before a near constant value is reached. These results indicate that any magnetic QCP in this system is well separated from strong valence fluctuations. The pressure dependence of the valence and lattice parameters of Yb2Ni12As7 are compared and at 1 GPa, there is an anomaly in the unit cell volume as well as a change in the slope of the Yb valence, indicating a correlation between structural and electronic changes.

铀化合物因为5f电子的多体相互作用而具有纷繁复杂的强关联特性，如非传统超导行为，磁有序行为和量子临界行为等。中等关联强度的反铁磁材料USb2（TN~201K）中的5f电子呈现出局域、巡游双重特性，该化合物具有准二维电子结构的特征，是研究关联效应较为理想的材料。我们将报道USb2的低温物理性质、扫描隧道显微镜（STM）和角分辨光电子能谱(ARPES)研究的最新结果。我们不但获得了USb2的原子分辨图像，而且获得了STS的温度演化规律： 温度影响较小的自旋密度波而形成的两个峰和随温度明显变化的一个杂化峰。 ARPES 结果表明USb2中呈现出多自由度耦合的特征：多条能带出现Kink，并发现了费米面嵌套及waterfall型能带结构特征。

高压下新型氢化物的形成与优异的超导电性

崔田

吉林大学超硬材料国家重点实验室，物理学院，长春，130012

金属氢是上个世纪没有解决的重大物理问题之一，理论曾预言超高压下氢分子晶体会转变成原子晶体，成为金属氢，并有望成为室温传统超导体，但迄今为止在实验可达到的压力范围内没有获得金属氢。在富氢分子晶体中，人们预期非氢元素的存在会对氢的子体系产生化学预压作用，这些体系比纯氢更容易金属化，并且具有象金属氢那样较高的超导转变温度。最近在新型氢化物的合成方面实验上取得了重要进展，为富氢化合物的研究提供了新的来源。

我们采用第一性原理计算方法结合晶体结构搜寻算法，发现发现了高压下可以使用碱金属俘获更多的氢分子，形成常压难以合成的富氢化合物KH₆，确定了高压结构和超导特性，160 GPa下T_c为65 K[1]；进一步又预言了在高压下(AlH₃)H₂形成新相，在250 GPa发生金属化，T_c高达130 K[2]。利用相似的学术思想，发现在高压下可以产生常规条件下难以产生的新型的硫氢化合物H₃S，明确了产生H₃S晶体的两个主要途径：3H₂S→2H₃S+S，2H₂S+H₂→2H₃S，即：可以对硫化氢直接加压到43 GPa或者通过对硫化氢和氢的混合物加压到3.5 GPa获得[3]。H₃S晶体能稳定存在到300万大气压，其它氢含量更高的H₄S、H₅S和H₆S化合物都不能稳定存在。针对新型硫氢化合物(H₃S)进行系统深入的研究，发现其在110万大气压下金属化，在200万大气压下的超导温度Tc高达191-204K[4]。进一步光谱函数的计算表明，它的超导温度主要是由氢原子振动贡献的。研究结果报道后不久，德国马普所的Eremets研究小组将H₂S样品加压到180万大气压以上，发现在温度达到190 K（-83℃左右）时变成超导体[5]，证实了我们的理论预言。

索引文献：

[1] D. Zhou, X. Jin, X. Meng, G. Bao, Y. Ma, B. Liu, T. Cui, Ab initio study revealing a layered structure in hydrogen-rich KH₆ under high pressure, Phys. Rev. B, 86 (2012) 014118.

[2] P. Hou, X. Zhao, F. Tian, D. Li, D. Duan, Z. Zhao, B. Chu, B. Liu, T. Cui, High pressure structures and superconductivity of AlH₃(H₂) predicted by first principles, Rsc Advances, 5 (2015) 5096-5101.

[3] D. Duan, X. Huang, F. Tian, D. Li, H. Yu, Y. Liu, Y. Ma, B. Liu, T. Cui, Pressure-induced decomposition of solid hydrogen sulfide, Physical Review B, 91(R), 180502 (2015).

[4] D. Duan, Y. Liu, F. Tian, D. Li, X. Huang, Z. Zhao, H. Yu, B. Liu, W. Tian, T. Cui, Pressure-induced metallization of dense (H₂S)₂H₂ with high-Tc superconductivity, Sci. Rep., 4, 6968 (2014).

[5] A.P. Drozdov, M. I. Eremets, I.A. Troyan, Conventional superconductivity at 190 K at high pressures, arXiv:1412.0460, (2014).

 

Recently, much attention has been paid to search for Majorana bound state (MBS) in solid-state systems. Among various searching proposals there is one based on radio-frequency superconducting quantum interference devices (rf-SQUIDs), in which a unique 4pi-perioded current-phase relation (CPR) is expected if MBS exists. Here we report our observation of two simultaneously-truncated patterns of contact resistance oscillation on Pb rf-SQUIDs constructed on the surface of three-dimensional topological insulator Bi₂Te₃. The results reflect the existence of a fully skewed CPR which is 4pi-perioded if without truncation. We ascribe the truncation to quasiparticle poisoning which happens unavoidably in our devices at every odd multiples of half flux quantum.

Projected Entangled Simplex State is a novel kind of tensor network states. It emphasizes the many-body simplex entanglement instead of the two-body pair entanglement, and is suitable to study the quantum frustrated systems. In this talk, I will give a brief introduction of this kind of wavefunction ansatz, and also show some recent result of its applications, e.g., Kagome (or triangle) anti-ferromagnetic Heisenberg model. 

     最近发现的过渡金属钛基磷族氧化物，ATi₂Pn₂O(A=Na，Ba；Pn=As，Sb)是一类结构与铜氧化物和铁基超导体类似的层状超导体，虽然在最佳掺杂的Ba_1-xNa_xTi_xSb_xO中Tc目前只有5.5 K，但作为新的一类二维层状超导体，它给人们提供了研究非传统超导体的新平台，引起了大家的广泛关注。与铁基超导体类似，在钛基磷族氧化物中也观察到了可能的电荷密度波（CDW）/自旋密度波（SDW）的特征。在非传统超导体中，这类有序态的失稳对Tc具有重要的作用，比如铁基超导体中的自旋涨落。因此，对钛基磷族氧化物中的密度波性质进行深入的研究具有重要的意义。Na₂Ti₂Sb₂O可以视为钛基超导体的母体材料，电阻率和磁化率测量表明Na₂Ti₂Sb₂O在115 K时存在一个相变，光谱测量结果发现在相变温度以下形成了密度波形式的能隙，但无法确定是SDW或者CDW能隙。因此，从电子结构的角度研究Na₂Ti₂Sb₂O中的密度波性质对我们理解这类材料中各种有序现象尤为重要。

       我们采用角分辨光电子能谱技术，对Na₂Ti₂Sb₂O的电子结构和密度波进行了详细深入的研究。实验测量得到的费米面和能带结构均与理论计算的非磁性态结果非常吻合，表明Na₂Ti₂Sb₂O的基态没有磁性而且关联性较弱。偏振依赖的光电子能谱结果表明Na₂Ti₂Sb₂O具有多能带、多轨道特性。kz依赖的光电子能谱结果表明Na₂Ti₂Sb₂O能带结构具有较弱的三维特性。在相变温度115 K之下，在布里渊区X点的空穴型能带上打开了密度波形式的能隙。我们的研究结果表明Na₂Ti₂Sb₂O是一类关联较弱的CDW材料。

Different from graphene which is orbitally inactive, the $p_x/p_y$-orbital bands in the 2D honeycomb lattice are orbitally active, which apply to both optical lattices and several classes of solid state systems including organic materials, fluoridated tin film, BiX/SbX(X=H, F, Cl, Br). The interplay between the orbital structure and spin-orbit coupling gives rise to the 2D quantum spin Hall state and quantum anomalous Hall state with large topological gaps. The gap magnitudes are equal to the spin-orbit coupling strength at the atomic level, and thus are much larger than those based on the s-p band inversion. The energy spectra and eigen-wavefunctions are solved analytically based on the Clifford algebra, which greatly facilitates the topological analysis. Flat bands also naturally arise and the consequential non-perturbative physics includes Wigner crystallization and ferromagnetism. In the Mott-insulating state, orbital exchange is highly frustrated described by a quantum 120$^\circ$ model which is similar to but different from the Kitaev model. An f-wave Cooper pairing arises if the band is filled with spineless fermions exhibiting boundary zero energy Andreev modes. Although the pairing mechanism is conventional, the unconventional pairing symmetry is driven by the non-trivial band structure.
Selected References
1. Gu-Feng Zhang, Yi Li, Congjun Wu, ,The honeycomb lattice with
multi-orbital structure: topological and quantum anomalous Hall insulators
with large gaps ,Phys. Rev. B 90, 075114 (2014).
2. Wei-cheng Lee, Congjun Wu, and S. Das Sarma, "$F$-wave pairing of cold
atoms in optical lattices", Phys. Rev. A 82, 053611 (2010).
3. Congjun Wu, "Orbital analogue of quantum anomalous Hall effect in
$p$-band systems", Phys. Rev. Lett. 101, 186807 (2008).
4. Congjun Wu, "Orbital orderings and frustrations of p-band systems in
optical lattices", Phys. Rev. Lett. 100, 200406 (2008).
5. Congjun Wu, Doron Bergman, Leon Balents, and S. Das Sarma, "Flat bands
and Wigner crystallization in the honeycomb optical lattice", Phys. Rev.
Lett. 99, 70401 (2008).

摘要：拓扑绝缘体作为一种新奇的凝聚态物质，其受时间反演对称性保护的表面态一直是人们研究的热点。目前为止，拓扑绝缘体表面态的研究绝大部分集中在对自然解理面的研究，而其他晶面因为具有较多的悬挂键而不稳定且难以制备，限制了人们对其的研究。非自然解理面方面的实验工作主要有Bi₂Se₃(221)表面表面态的高度各项异性研究[1]、Bi_1-xSb_x(110)表面三个狄拉克锥的实现[2]及其表面态稳定性研究[3]、以及Bi₂Se₃(001)面拓扑悬挂键的研究[4]。

在本工作中，我们利用分子束外延在Si(111)衬底上进行了高质量拓扑绝缘体Bi₂Te₃薄膜的生长，利用低温扫描隧道显微术，我们对其表面形貌以及电子结构进行了表征。我们发现样品表面除了绝大部分为整数原胞层厚外,还存在少量分数原胞层厚的区域。从扫描隧道谱来看，台阶高度为单层原子厚度的区域表现出与其他区域截然不同的电子性质。我们利用第一性原理计算，模拟了不同分数原胞层厚表面的电子结构并与实验结果进行了比较。此工作对深入理解拓扑绝缘体表面态拓扑性质及其稳定性具有重大意义。

Abstract: 

Topological insulators (TIs) belong to a novel phase of condensed matter and much effort has been devoted to the research on the topological nature of the surface states of TIs. Most works focus on the natural cleaving surface of TIs. However, researches on the non-natural surfaces of TIs are hindered due to the difficulties in preparation of those surfaces and the existence of large number of dangling bonds on those surfaces. A quite limited number of works have been done, trying to illuminate the wonderland of those special surfaces of TIs [1-4].

In this work, using the state-of-art molecular beam epitaxy (MBE), we manage to prepare topological insulator Bi₂Te₃ thin films with fractional quintuple layer (FQL) termination. Scanning tunneling microscopy (STM) reveals that the as-grown Bi₂Te₃ thin films may not necessarily terminate at the Van der Waals gap between two adjacent quintuple layers. The electronic structures of the FQL surfaces are studied in combination with quasi-particle interference (QPI) by scanning tunneling spectroscopy (STS), surface state (SS) calculations by tight binding method based Green’s Function and density functional theory. Our results suggest that the topological nature of SSs be preserved on different FQL terminations. The robustness of the topological SSs is also demonstrated.

We first predict that the recently discovered quasi-one dimensional superconductors, A₂Cr₃As₃(A=K,Rb),  possess  strong frustrated magnetic fluctuations and are nearby a novel in-out co-planar magnetic ground state. Then, we show that a minimum three-band model based on the d_z²,d_xy and d_x²-y² molecular orbitals of one Cr sublattice can  capture the band structures near Fermi surfaces. In both weak and strong coupling limits, the standard random phase approximation (RPA) and mean-field solutions consistently yield the triplet p_z-wave pairing as the leading pairing symmetry for physically realistic parameters. The triplet pairing is driven by the ferromagnetic fluctuations within the sublattice. The gap function of the pairing state possesses line gap nodes on the k_z=0 plane on the Fermi surfaces. Finally, we investigate the experimental consequences of pz-wave spin triplet superconductivity in A₂Cr₃As₃.

 

 

 

 

Reference

[1] X. X. Wu, C. C. Le, J. Yuan, H. Fan and J. P. Hu, Chin. Phys. Lett. 32 057401(2015).

[2] X. X. Wu, F. Yang, C. C. Le, H. Fan and J. P. Hu, arXiv: 1503.06707(2015).

[3] X. X. Wu, et al. (in preparation)

The formation of heavy fermion bands can occur by means of the conversion of a periodic array of local moments into itinerant electrons via the Kondo effect and the huge consequent Fermi-liquid renormalizations. Leggett predicted for liquid ³He that Fermi-liquid renormalizations change in the superconducting state, leading to a temperature dependence of the London penetration depth lamda quite different from that in the BCS theory. Using Leggett's theory, as modified for heavy fermions, it is possible to extract from the measured temperature dependence of lamda in high quality samples both Landau parameters F₀^S and F₁^S; this has never been accomplished before. A modification of the temperature dependence of the specific heat C_el , related to that of lamda, is also expected. We have carefully determined the magnitude and temperature dependence of lamda in CeCoIn₅ by muon spin relaxation rate measurements to obtain F₀^S=36±1 and F₁^S=1.2±0.3 , and find a consistent change in the temperature dependence of electronic specific heat . This, the first determination of F₁^S with a value ≤ F₀^S in a heavy fermion compound, tests the basic assumption of the theory of heavy fermions, that the frequency dependence of the self-energy is much more important than its momentum dependence.

 

The filled skutterudites MT₄X₁₂ (M: rare-earth or alkaline earth metals; T: transition metal; X: pnicogen) display rich physical properties, among which the filled skutterudite superconductors are of much attention [1][2][3]. Unconventional superconductivity in the novel filled skutterudite superconductor is often characterized by broken symmetries such as time reversal symmetry (TRS). ZF-µSR technique has been proved to be quite powerful in revealing broken TRS by signaling the appearance of a static internal magnetic field B_s in the superconducting regime [4]. In the most investigated PrOs₄Sb₁₂ and PrPt₄Ge₁₂, multiple superconducting phases are generated by the delicate correlation between the Pr³⁺ crystal electric field (CEF) and the hybridization of the f shell with the conduction electrons at the host atom [1], making doping effect a fascinating approach to lay more insights on the evolution of TRS in these compounds [2][5]. Comparison between different rare-earth doping effects will be discussed in detail.

Reference

[1]A. Maisuradze, et al, Phys. Rev. B 82, 024524 (2010)

[2]L. Shu, et al, Phys. Rev. B 83, 100504(R) (2011)

[3]Y. Aoki, et al, Phys. Rev. Lett. 91, 067003 (2003)

[4]J. H. Brewer, Encyclopedia of Applied Physics, edited by G. L. Trigg (VCH, New York, 1994), Vol. 11, p. 23.

[5]J. Zhang, et al, Phys. Rev. B 91, 104523 (2015)

三维狄拉克半金属受对称性保护。我们提出了一个利用时间反演对称性、镜面对称性以及旋转对称性对稳定三维狄拉克半金属分类的新框架，据此我们发现五个种类的镜面一共可以产生出两类狄拉克点及狄拉克线。第一类狄拉克点是通过偶然能带简并实现的，而第二类狄拉克点通过线性能带交叉实现，后者包含受晶体对称性保护的类型并且预示着新型的狄拉克半金属。在我们的框架之下，我们还证明了非平凡镜面对称性保护的狄拉克线的存在。

We study the Landau-Zener-St\"{u}ckelberg interference in a Majorana qubit and show that it operates the qubit controllably. The Majorana qubit consists of an rf SQUID with a topological nanowire Josephson junction which hosts Majorana bound states. We consider an asymmetric triangular magnetic flux pulse through the SQUID, which enforces the quantum evolution of the Majorana qubit. The Majorana qubit experiences two Landau-Zener transitions when the amplitude of the pulse is tuned around the superconduction flux quanta $2e/\hbar$. The resulted Landau-Zener-St\"{u}ckelberg interference rotates the Majorana qubit, where the rotation angle is controlled by varying the time scale of the pulse. This rotating operation implements a single-qubit gate for the Majorana qubit, which is a necessary ingredient for the topological quantum computation.

Manipulating the spin states of particles has triggered many innovational fields such as spintronics and topological insulators which focus on the single electron spin. Besides electron, Cooper pairs also have their spin states which are known as spin-singlet and spin-triplet states. In this talk, I first give a unified theory of the spin-singlet and spin-triplet Cooper pairs in the presence of either magnetization or spin-orbit coupling. Then I will show that Majorana fermions on the boundary of topological superconductors only have spin-triplet superconducting correlation. This is universal for all TSCs. This spin-triplet condensate results in the spin-orbit coupling (SOC) controlled oscillatory critical current without 0-π transition in the TSC/SOC-semiconductor/TSC Josephson junction. The observation of this unique current-phase relation can serve as the signal of Majorana fermions. Moreover our study open a new way to manipulate Majorana fermions based on their spin-triplet superconducting correlation.

We study a simple two-orbital t-J1-J2 model for iron-based superconductors in the presence of a bond disorder (via nearest-neighbor bond-dilution). By using Bogoliubov-de Gennes approach, we self-consistently calculate the local pairing amplitudes and the corresponding density of states, which demonstrate a change of dominant pairing symmetry from the s+- wave to d-wave as long as J2~J1. Although the system exhibits spatially inhomogeneous pairing in weak correlations with a given realization of disorder, it is still in sharp contrast to the case with potential disorder, where the superconducting islands and the insulating sea are both present in the strong disorder regime. Moreover, from the detailed examination of the pairing gap as well as the superfluid density, the superconducting transition here is suggested to be beyond the conventional Abrikosov-Gorkov consideration.

Reference: Yao-Tai Kang, Wei-Feng Tsai, Dao-Xin Yao, to be submitted

我们通过化学气相沉积法合成了各种Bi₂Se₃纳米结构。在低温强磁场条件下系统研究了单个 Bi₂Se₃纳米片的拓扑绝缘体输运性质，发现其载流子迁移率高达10000 cm2/Vs，并且在14 特斯拉强磁场下观察到高达400%尚无饱和迹象的正磁阻效应 (MR)。通过研究磁电阻对磁场的角度和温度的依赖关系，我们揭示了这种线性的MR起源于二维输运机制，其大小与载流子迁移率呈正相关性。[1]同时，在Bi₂Se₃纳米片和纳米带的量子输运测量中观测到Shubnikov-de Haas (SdH)振荡，分析表明存在Berry相位π，这是拓扑绝缘体表面狄拉克费米子的重要特征。[2]然而，在垂直磁场下，SdH振荡总是叠加在一个很大的线性磁阻背景上，这为分析表面狄拉克费米子的输运行为带来困难。我们通过外加平面内磁场，测量到没有正磁阻背景的SdH振荡，理论分析确认了来源于Bi₂Se₃纳米片侧面表面态的量子输运。[3]

通过圆偏振光可以选择性激发拓扑绝缘体的表面态，从而产生自旋极化的表面态，由于自旋方向与动量方向的锁定关系，样品中会产生定向运动的光电流。Bi₂Se₃也是一个很好的热电材料，在非均匀光的辐照下，会产生光热电效应。我们在实验上观测到Bi₂Se₃中圆偏振光激发增强的光热电效应，该结果有望用于自旋极化的电流源。[4]

参考文献：

[1] Yuan Yan, Li-Xian Wang, Da-Peng Yu, Zhi-Min Liao. Large magnetoresistance in high mobility topological insulator Bi2Se3. Applied Physics Letters 103, 033106 (2013).

[2] Yuan Yan, Zhi-Min Liao, Yang-Bo Zhou, Han-Chun Wu, Ya-Qing Bie, Jing-Jing Chen, Jie Meng, Xiao-Song Wu, Da-Peng Yu. Synthesis and Quantum Transport Properties of Bi2Se3 Topological Insulator Nanostructures. Scientific Reports 3, 1264 (2013).

[3] Yuan Yan, Li-Xian Wang, Xiaoxing Ke, Gustaaf Van Tendeloo, Xiao-Song Wu, Da-Peng Yu, Zhi-Min Liao. High-Mobility Bi2Se3 Nanoplates Manifesting Quantum Oscillations of Surface States in the Sidewalls. Scientific Reports 4, 3817 (2014).

[4] Yuan Yan, Zhi-Min Liao, Xiaoxing Ke, Gustaaf Van Tendeloo, Qinsheng Wang, Dong Sun, Wei Yao, Shuyun Zhou, Liang Zhang, Han-Chun Wu, Dapeng Yu. Topological Surface States Enhanced Photothermoelectric Effect in Bi2Se3 Nanoribbons. Nano Letters 14, 4389 (2014).

The interacting bosons in one-dimensional inversion-symmetric superlattices are investigated from the topological aspect. The complete phase diagram is obtained by an atomic-limit analysis and quantum Monte Carlo simulations and comprises three kinds of phases: superfluid, persisted charge-density-wave and Mott insulators, and emergent insulators in the presence of nearest-neighbor hoppings. We find that all emergent insulators are topological, which are characterized by the Berry phase π and a pair of degenerate in-gap boundary states. The mechanism of the topological bosonic insulators is qualitatively discussed and the ones with higher fillings can be understood as a 13-filling topological phase on a background of trivial charge-density-wave or Mott insulators.

 

References: 

1: Tianhe Li, Huaiming Guo, Shu Chen, and Shun-Qing Shen, Phys. Rev. B 91, 134101 (2015).

Carbon, one of the most abundant elements in the universe, has never ceased to amaze and surprise us. In this talk, I will discuss how to arrange graphene sheets into three-dimensional structures to achieve unexpected physical properties with new topological classifications. Key in realizing such structures is, of course, the ability to control the interplay between carbon sp² and sp³ hybridization, which, at this point, is still an experimental challenge.Regardless, I will show that an interpenetrated graphene network (IGN), which is only 0.1 eV/C higher in energy than diamond, exhibits Weyl loops as the Fermi surface. Upon truncation into a layered structure of few-layer thickness, a nearly flat band structure at the Fermi level with huge density of states emerges, indicating that the slab maybe can serve as superconductors. The relationship between the Weyl loops and a set of spin-orbit-free Weyl points is established by reducing the crystal symmetry of the IGN, as expected from a model Hamiltonian analysis. Moreover, upon applying a uniaxial stress, the IGN can be continuously transformed into a three-dimensional carbon Kagome lattice (CKL) [Y. Chen, et al., Phys. Rev. Lett. 113, 085501 (2014)]. The CKL is predicted to have a direct band gap in the visible blue range, due to the orbital frustration physics unique to triangular lattice. The calculated effective masses of the CKL are comparable to those of silicon, while its optical properties are comparable to gallium nitride and zinc oxide, which points to a new direction in designing optoelectronic materials out of elemental semiconductors.

 

The fractional quantum Hall droplet can undergo a series of phase transitions on the cylinder geometry via varying the aspect ratio in a finite size system. Since the distance between two ends is linear as a function of the system size and can be easily tuned by varying the aspect ratio, we systematically study the quasiparticle tunnelling amplitudes, the wave function overlap, bipartite entanglement and the electron Green's function of the Laughling state at ν=1/3. Two critical length scales of the edge-edge distance are found to be related to the dimension reduction and back scattering point respectively.

The low energy physics of one-dimensional (1D) many-body  systems  usually refers to the Luttinger liquid theory--collective motion of bosons, which is significantly different from the behaviour of quasiparticles in the Fermi liquid in higher dimensions. Despite  quite different microscopic origins of various types of quantum  liquids, many of their macroscopic properties are  common. In this talk we demonstrate that the conceptually  simple  Wilson  ratio  reveals  the  essence of FLs and TLLs, as well as  manifests   the origin of the breakdown of these liquids near critical points. We find that the thermodynamic quantities, e.g.,   inverse susceptibility $1/\chi$, compressibility $\kappa$ and specific heat $c_v$   in a $w$-component liquid simply sum up  these of individual liquids. The Wilson ratio, describing particle  number fluctuation,   displays plateaus of integers $1, 2^2,..., w^2$ that significantly identify  different states of quantum  liquids    in strongly attractive   multicomponent fermions.  Whereas, in the scenario of spin-charge separation, the Wilson ratio, describing the spin fluctuation,   displays the plateaus of  $2, 4,..., w(w+1)/3$ for the $2, 3, \ldots, w$-component strongly repulsive fermions, respectively. The Wilson ratio provides a measurable parameter for experimentally determining quantum liquids of this kind at the renormalization fixed point.  In addition, we also show that, near a continuous classical or quantum phase transition, Tan's contact exhibits a variety of critical behaviors, including scaling laws and critical exponents.
%

In this talk, we will introduce our magnetoresistance study on SmB₆ at low temperature up to 55 Tesla[1]. Both temperature- and angular-dependent magnetoresistances show a similar crossover behavior below 5 K. Furthermore, the angular-dependent magnetoresistance on different crystal face confirms a two-dimensional surface state as the origin of magnetoresistances crossover below 5K. Based on two-channels model consisting of both surface and bulk states, the field-dependence of bulk gap with critical magnetic field (H_c) of 196 T is extracted from our temperature-dependent resistance under different magnetic fields. Our results give a consistent picture to understand the low-temperature transport behavior in SmB₆, consistent with topological Kondo insulator scenario. In addition, we will also introduce our recent results on Yb doped Kondo insulator SmB₆. Through NMR and specific heat measurements, we observed a systematic bulk electronic evolution for Yb doped SmB₆ and a possible heavy fermion phase appears in heavily doped SmB₆ samples. Finally, a possible global phase diagram has been proposed for Sm_1-xYb_xB₆.

 

[1] F. Chen et al., Physical Review B 91, 205133 (2015)

我们首先介绍典型重费米子体CeCu2Si2的低温热电效应和能势特效应，分析重费米子异常热输运，异常霍尔效应，和局域近藤散射的密切关系，讨论近藤体系的巨大热电信号的微观起源。通过讨论几个典型重费米子体系的热电和能势特效应，确立能势特效应作为一个研究关联体系电子弛豫过程的手段。最后，介绍我们最近制备的几个新型近藤体系的异常电和热输运现象。

We study the fate of double-Weyl fermions in three-dimensional systems in the presence of long- range Coulomb interactions. By employing the momentum-shell renormalization group (RG) approach, we find that the fixed point of non-interacting double-Weyl fermions is unstable against Coulomb interactions and flows to a non-trivial stable fixed point with anisotropic screening effect. Moreover, experimentally-measurable quantities such as specific heat obtain unusual logarithmic corrections. We also discuss implications of our results to three-dimensional materials HgCr2Se4 and SrSi2, candidate materials of hosting double-Weyl fermions.

We investigate the influence of different spin–orbit couplings on topological phase transitions in the bilayer Kane–Mele model. Wefind that the competition between intrinsic spin–orbit coupling and Rashba spin–orbit coupling can lead to two dimensional topological metallic states with nontrivial topology. Such
phases, although having a metallic bulk, still possess edge states with well defined topological invariants. Specifically, we show that with preserved time reversal symmetry the system can exhibit a2-metallic phase with spin helical edge states and a nontrivial 2 invariant. When time reversal symmetry is broken, a Chern metallic phase could appear with chiral edge states and a nontrivial Chern invariant.

50年前，Fulde-Ferrell以及Larkin-Ovchinnikov分别理论预言了具有超导序参量的位相或模值在实空间周期性分布的极化费米子超导态。近年来，人们注意到具有自旋轨道耦合的BCS超导体在外磁场的作用下能够进入到拓扑超导态。具体的，在连续体系中理论上发现了具有非平庸拓扑属性的FF态，其超导位相在实空间周期性分布并且其Chern数为1。我们在二维的格点模型中，通过计算体系的Chern数、能谱特性、序参量、零能态密度谱等，得到了可以稳定存在的4种具有非平庸拓扑属性的FFLO态，分别为两种拓扑FF态（Chern数为1或2），两种拓扑LO态（Chern数为1或2）。

本工作将介绍具有双螺旋磁性的CrAs和MnP的晶体结构与压力之间的关系。CrAs与MnP都具有相同的晶体结构，基态都具有双螺旋磁性，常温下他们的a轴和c轴都会随压力增大先增加后减小。不同之处在于CrAs在0.4Gpa有一个塌缩相，而MnP在8Gpa（超导压力）区间没有发生相变，而在12.5Gpa发生一个结构相变。

 

Water is one of the most important chemical substances on Earth. For its crucial role in many biological, chemical and environmental processes, understanding its microscopic structure is an issue of long-standing interest. The simulation of liquid water has been so far applied by ab initio molecular dynamics within the DFT framework, revealing the difficulty of current density functionals to unambiguously provide structural features which agree with experimental observations, despite the inclusion of empirical corrections. The need for higher-level and empiricism-free reference simulations is therefore of great importance to provide a fully ab initio prediction of the water structure. In our recent work, we performed the first ab initio molecular dynamics simulation of liquid water with forces evaluated by means of quantum Monte Carlo (QMC), one of the most accurate methods for calculating electronic properties of many-body systems. The simulated properties are in good agreement with recent neutron scattering and X-ray experiments, particularly concerning the position of the oxygen-oxygen peak in the radial distribution function. Given the excellent performance of QMC algorithms on peta-scale supercomputers and the good scaling with the system size, we've opened promising perspectives for predictive and reliable ab initio simulations of challenging systems in the future.

In Kondo insulator samarium hexaboride SmB6, strong correlation and band hybridization lead to a diverging resistance at low temperature. The resistance divergence ends at about 3 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been observed to map the Fermi surface. We solve the problem by resolving the Landau Level quantization and Fermi surface topology using torque magnetometry. The observed angular dependence of the Fermi surface cross section suggests two-dimensional surface states on the (101) and (100) plane. Furthermore, similar to the quantum Hall states for graphene, the tracking of the Landau Levels in the infinite magnetic field limit points to -1/2, the Berry phase contribution from the 2D Dirac electronic state. Finally we will discuss on the progress on quantum oscillations on the floating-zone-method-grown ultrahigh quality SmB6 single crystals.

使用精准控制表面和界面的方法来制备高质量的氧化物薄膜与超晶格是氧化物电子学的材料基础。本报告主要汇报我们在钙钛矿氧化物超晶格及异质界面的一些研究进展，特别是结合脉冲激光沉积（PLD）、第一性原理计算和球差电镜技术等多种先进手段所取得的一些成果，重点介绍BiFeO₃/LaAlO₃界面的极化行为和Bi₂FeMnO₆超晶格的相关结果。我们希望这些研究有助于深度理解氧化物界面及超晶格的物性特别是层展现象（emergent phenomenon ），也有利于未来氧化物电子器件走向实用化。

The orthorhombic structure MnP is a newly discovered superconductor under 7 _ 8 GPa. We

present the optical conductivity study of MnP single crystal, at different temperatures crossing

the heli-ferro-paramagnetic transitions. A Drude-like peak is observed from 300 K to 10 K.

Detailed analysis reveals that there are two kinds of carriers with different life times. In helimagnetic

phase, new interband transitions appear. The relationship between the localized carriers

and the helimagnetic ordering at 10 K is discussed.

By first-principles calculation based on density functional theory (DFT) with modified Becke
and Johnson potentials plus Hubbard U (MBJLDA+U), we studied the electronic structures of
antiferromagnetic (AFM) GdBiPt with propagating vectors Q⃗1=(0,0,π)(A-type) and Q⃗2=(π,π,π)(Gtype).
By the total energy calculation, the G-type AFM spin ordered state is relatively stable than
the A-type AFM spin ordered state, though the difference of total energy is minute. Our band
structure calculation predicts that the A-type AFM state is topological nontrivial due to a single s-
character band inversion at Γ point, which is similar to the band inversions in half-Heusler topological
insulator candidates and bulk HgTe semiconductor, while the G-type AFM state is topologically
trivial due to the absence of s/p band inversion. By a realistic tight-binding model calculation with
twenty bands coupled to a AFM Zeeman field, GdBiPt with A-type AFM spin order present metallic
surface state on the terminations with AFM aligned Gd ions, and this surface state is independent
with the strength of AFM Zeeman field, i.e. this surface state will preserve to nonmagnetic case. On
the termination with ferromagnetic (FM) spin aligned Gd ions, the surface state is dependent with
the strength of Zeeman field, and metallic surface can recover when the Zeeman field approaches to
zero.

We study spin reorientation phenomena and interaction e ects under various applied strain on the (001) surface of topological crystalline insulators, Pb$_{1-x}$Sn$_x$(Te, Se), which host multiple Dirac cones at low energies. By using a four-band k
p model, which captures the spin and orbital texture of the surface states well around X (or  Y ) up to the energy above Lifshitz transition, we find that even without time-reversal symmetry breaking, the hedgehog-like spin texture associated with a gap formation can be induced by certain strain, breaking one of the mirror symmetries; while the other cases cannot induce a gap at Dirac points. Furthermore, through the interplay between the perpendicular Zeeman eld and the strain, we demonstrate that the Chern number of the gapped surface ground state is tunable, resulting in great potential for power-saving electronics. Finally, we also investigate the strain effect for the interaction-driven competing orders and obtain the evolution of the phase diagram at the mean- eld level and discuss its possible applications benefiting fromelectron correlations.

Iron based superconductors and topological insulators(TI) are two types of intriguing materials with both important fundamental physics and potential revolutionary applications. Here, we report our recent research in two kinds of iron based superconductors FeTeSe and CaFeAs2 which both integrate quantum spin hall(QSH) effect and high temperature superconductivity(SC). We find that in single layer FeTeSe, its Z₂ topological invariance is completely determined by the parity at /gama point in the Brillouin zone(BZ). The single layer FeTeSe can go through a topological phase transition by adjusting the height of the Se(Te) atom which can be realized as a function of the Se(Te) component. CaFeAs2 has a structure with staggered CaAs and FeAs layers. While the FeAs layers are known to be responsible for SC, we show each CaAs layer is in a Z₂ nontrivial state, which leads to a fact CaFeAs2 is a natural material where a two dimensional(2D) TI is in close proximity to a superconductor. This gives us an unique opportunity to realize majorana modes in real materials.

 

 

Reference

[1] X. X. Wu, S. S. Qin, Y. Liang, H. Fan and J. P. Hu, arXiv: 1412.3375

[2] X. X. Wu, S. S. Qin, Y. Liang, C. C. Le, H. Fan and J. P. Hu, Phys. Rev. B. 91,081111

Based on first-principles calculations, we have found that a family of two-dimensional transition-metal
dichalcogenide haeckelites with square-octagonal latticeMX2-4-8 (M = Mo,W and X = S, Se and Te) can host
quantum spin Hall effect. The phonon spectra indicate that they are dynamically stable, and the largest band gap
is predicted to be around 54 meV. These will pave the way for potential applications of topological insulators.We
have also established a simple tight-binding model on a squarelike lattice to achieve a topologically nontrivial
quantum state, which extends the study from honeycomb lattice to squarelike lattice and broadens the promising
topological material systems greatly.

The surface states of three-dimensional topological insulators provide a fertile ground to explore the localization phenomena. One such example is Anderson localization, the absence of diffusive transport in disordered systems, which has profoundly influenced our understanding of electron conductivity or more generally the behavior of waves in various media. While Anderson localization is manifested as hopping transport in numerous electronic systems, it remains elusive in the recently discovered topological insulators. In this talk we present an experimental demonstration of a crossover from diffusive transport in the weak antilocalization (WAL) regime to variable range hopping transport in the Anderson localization regime with ultrathin (Bi_1-xSb_x)₂Te₃ films. As disorder becomes stronger, the negative magnetoconductivity corresponding to the WAL is gradually suppressed, and eventually, positive magnetoconductivity emerges when the electron system becomes strongly localized. This work reveals the critical role of disorder in the quantum transport properties of ultrathin topological insulator films, in which theories have predicted rich physics related to topological phase transitions.

Orbital-selective Mott phase driven by peculiar orbital order in iron-pnictide superconductor LaFeAsO_1-xH_x

Da-Yong Liu ^1,2, Feng Lu ³, Xiang-Long Yu ¹ , Wei-Hua Wang ³, and Liang-Jian Zou ^1,2∗

1 Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, P. O. Box 1129,   Hefei, Anhui 230031, China

  2 Department of Physics, University of Science and Technology of China, Hefei, 230026, China

3 Department of Electronics & Tianjin Key Laboratory of Photo-Electronic Thin Film Device and Technology, College of     Electronic Information and Optical Engineering, Nankai Uniersity, Tianjin 300071, China

Abstract:

       Recent experiments reported that the phase diagram of hydrogen doped LaFeAsO_1-xH_x demonstrates double dome superconducting phases related with bipartite magnetic parent phases (AF1-SC1-SC2-AF2 phase diagram), different from other iron-based superconductors. Nonetheless, the origin of the second superconducting/magnetic phase in LaFeAsO_1-xH_x remains unclear.

       In this work, we investigate the electronic structure and magnetism of LaFeAsO_1-xH_x using the first-principle calculations combined with analytic methods. We find that the magnetic ground state of the parent phase La₂Fe₂As₂OH (LaFeAsO_0.5H_0.5) is a stripe-antiferromagnetic (SAFM) phase with a large next nearest-neighboring magnetic exchange constant J₂ in an effective J_1a-J_1b-J₂ Heisenberg model. Within a five-orbital Hubbard model, it is found that the unique magnetism is associated with a peculiar xy ferro-orbital order ascribing to an unusual FeAs tetrahedral lattice distortion. Furthermore, this orbital order drives La₂Fe₂As₂OH into an orbital-selective Mott phase (OSMP), where the xy orbital is nearly insulating, resulting in emergence of the second novel magnetic parent phase. All these results provide a fundamental understanding on the origin of the two distinct magnetic/superconducting phases in LaFeAsO_1-xH_x compounds.

众所周知，只要时间反演和电荷守恒对称性不被破坏，拓扑绝缘体的边界将维持为无能隙的导电状态。类似的现象也存在于玻色性系拓扑绝缘体中。玻色性拓扑绝缘体是一种非平凡的Mott绝缘体。除了对称性保护的无能隙边界激发外，其另一个标志性特征是体内的pi通量会trap住一个时间反演对称性保护的Kramers doublet。玻色拓扑绝缘体是对称性保护拓扑相的一个特例。近几年文献中对玻色性拓扑绝缘体的分类和物理性质有大量的讨论，但目前仍然很少有微观的格点模型实现这些量子相。为此， 我们提议了一种简单的机制来实现玻色拓扑绝缘相，并且在玻色系统或自旋系统中构造了微观格点模型来实现玻色性拓扑绝缘量子态。我们的方法可以用来实现其更多的他对称性保护的拓扑绝缘相。

Non-centrosymmetric transition metal monopnictides, including TaAs, TaP, NbAs and NbP, have been very recently proposed to be Weyl semimetals (WSMs), in which the low-energy quasiparticle excitations are the condensed-matter-physics realization of the long-sought-out Weyl fermions, which previously only appear in theoretical high-energy physics. The first fundamental question in this emergent field is that which compound is the ideal platform for exploring various fascinating quantum phenomena and novel device concepts. Although chiral WSM states have been observed in TaAs and NbAs, there is no conclusive evidence on the existence of Weyl fermions in NbP. Here, we use angle-dependent quantum oscillations to reveal that NbP has four pairs of unusually large Weyl fermion pockets in the k_z=0 plane near the high symmetry points Σ, dominating over the coexisting massive hole pockets and the previous reported WSM pockets in the k_z=1.18π/c plane. Such dominant WSM pockets are highly anisotropic in k-space and approaching the parabolic band top along the internode direction. The corresponding Fermi surface is consisting of helical Weyl fermions with unprecedented mobility of 1×10⁷ cm²V^-1s^-1 at 1.5 K, well protected from defect backscattering by real spin conservation associated to the chiral Weyl nodes. Inter-pocket pumping of Weyl fermions with opposite helicity becomes feasible when the magnetic field and electric field are applied in parallel, manifested as robust chiral anomaly induced negative MR. Surprisingly, we have observed a strong offset to the π Berry’s phase, which may be correlated to the non-negligible mass of Weyl fermions.

超导量子比特是目前重要的研究方向之一，其显著优点包括良好的工艺可扩展性、单比特的精确可操控性以及比特间的易耦合性等。但其技术上难以获得足够长的退相干时间，特别是在多比特复杂的量子系统的情况下。而固态中的自旋系统则通常具有很长的退相干时间，但工艺上却难以加工扩展到多比特系统。因而将两者结合起来形成的量子混合系统则有可能取长补短，最终朝着实用的量子计算前进。 本次报告将介绍我们在磁通量子比特与金刚石中的氮-空穴（NV）色心的量子杂化系统方面取得的一些进展，主要包括能隙可调型的磁通量子比特与NV色心系综的相干耦合，在该混合系统中演示的单比特量子存储操作，以及我们在努力提高存储寿命方面的一些努力。

Superconducting transitions under high pressure have been measured in LaRu₂P₂ which has an iso-structure of the iron-based 122 superconductors. An enhancement of Tc from 3.84K to 5.77K is discovered by using a pressure of only 1.74 GPa. The ab-initio calculation shows that the superconductivity in LaRu₂P₂ at ambient pressure can be explained by the McMillan^'s theory with strong electron-phonon coupling. However, it is difficult to interpret the pressure induced enhancement of T_c with this picture. Detailed analysis of the pressure induced evolution of resistivity and upper critical field H_c2(T) reveals that the increases of T_c with pressure may be accompanied by the involvement of extra electronic correlation effect. This suggests that the Ru-based system has some commonality as the Fe-based superconductors.

To explore new superconductors beyond the copper-based and iron-based systems is very important. The Ru element locates just below the Fe in the periodic table and behaves like the Fe in many ways. One of the common thread to induce high temperature superconductivity is to introduce moderate correlation into the system. Here we report the significant enhancement of superconducting transition temperature from 3.84K to 5.77K by using a pressure only of 1.74 GPa in LaRu₂P₂ which has an iso-structure of the iron-based 122 superconductors. The ab-initio calculation shows that the superconductivity in LaRu₂P₂ at ambient pressure can be explained by the McMillan's theory with strong electron-phonon coupling. However, it is difficult to interpret the significant enhancement of T_c versus pressure within this picture. Detailed analysis of the pressure induced evolution of resistivity and upper critical field H_c2(T) reveals that the increases of T_c with pressure may be accompanied by the involvement of extra electronic correlation effect. This suggests that the Ru-based system has some commonality as the Fe-based superconductors.  

To explore new superconductors beyond the copper-based and iron-based systems is very important. The Ru element locates just below the Fe in the periodic table and behaves like the Fe in many ways. One of the common thread to induce high temperature superconductivity is to introduce moderate correlation into the system. Here we report the significant enhancement of superconducting transition temperature from 3.84K to 5.77K by using a pressure only of 1.74 GPa in LaRu₂P₂ which has an iso-structure of the iron-based 122 superconductors. The ab-initio calculation shows that the superconductivity in LaRu₂P₂ at ambient pressure can be explained by the McMillan's theory with strong electron-phonon coupling. However, it is difficult to interpret the significant enhancement of T_c versus pressure within this picture. Detailed analysis of the pressure induced evolution of resistivity and upper critical field H_c2(T) reveals that the increases of T_c with pressure may be accompanied by the involvement of extra electronic correlation effect. This suggests that the Ru-based system has some commonality as the Fe-based superconductors.

In our recent work, we discovered a new topological phase of matter, the valley-polarized quantum anomalous Hall (VQAH) phase. The hallmark of this phase is that at system edges where valleys can be distinguished. There exist unbalanced numbers of counter-propagating chiral edge channels associated with different valley indices. We analyzed the necessary mechanism for generating VQAH, and provide a clear physical picture of the topological phase transition from conventional QAH phase to the VQAH phase. We show that with short-range disorders, pairs of counter-propagating edge channels from both valleys could be destroyed. However, at moderate scattering strength, the transport coefficients exhibit a plateau on which the transport at each edge is fully valley-filtered. This remarkable effect could be used for designing perfect valley filters for valleytronics applications. We further show that bilayer system has a much richer phase diagram with multiple VQAH phases. We demonstrate the controllability of the topological phase transition, especially by tuning the interlayer bias potential.

We investigated the form of the orbital ordering in the electronic nematic phase of the Iron-based superconductors by applying a group theoretical analysis on a realistic five-band model. We find the orbital order can be either of the inter-orbital s-wave form or the intra-orbital d-wave form. From the comparison with existing ARPES measurements of band splitting, we find the orbital ordering in the 122 system is dominated by an intra-orbital d-wave component, while that in the 111 system is dominated by an inter-orbital s-wave component. We find both forms of orbital order are strongly entangled with the nematicity in the spin correlation of the system. The condensation energy of the magnetic ordered phase is found to be significantly improved(by more than 20 percents) when the degeneracy between the (π,0) and (0,π) ordering pattern is lifted by the orbital order. We argue there should be large difference in both the scattering rate and the size of the possible pseudogap on the electron pocket around the X=(π,0) and Y=(0,π) point in the electronic nematic phase. We propose this as a possible origin for the observed nematicity in resistivity measurements.

Chaos and Integer Quantum Hall effect (IQHE) are thought to be unrelated back in old days. Here we present a first-principle analytic theory showing that in a large class of spin-1/2 quasiperiodic quantum kicked rotors, a dynamical analog of integer quantum Hall effect may emerge from intrinsic chaotic structure, which brought chaos and IQHE together. Specifically, the inverse of the Planck's quantum (he) and the rotor's angular momentum variance (AMV) growth rate mimic the filling fraction and the longitudinal conductivity in conventional IQHE, respectively; while a hidden quantum number is found to mimic the quantutized Hall conductivity. Two distinct "phase" are found. One is "critical metal" with unbounded AMV growth but universal growth rate at a serial of discrete he while the other is "Anderson insulator" with increasing topological number with a bounded AMV by decreasing Planck's quantum. The most impressive point for this new phenomenon is the missing of Fermi statistics and Landau band filling which may indict a different mechanism other than canonical TKNN frame. This phenomenon is sensitive to chaotic structure while it disappears when regular dynamics is partially recovered. To summarize, this new topological effect in dynamical chaotic system is beyond the Floquet band thoery and may deepen our understanding for IQHE.  

Hexagonal　boron nitride (h-BN), as a dielectric substrate, can largely preserve the superior properties of graphene. Vertical or lateral graphene/hBN hetero-structures may bring in opportunities for novel electronic devices. Scalable synthesis of high quality single crystalline hBN, graphene and graphene/hBN are strongly desired, but remains to be extremely challenging. In this talk, I will present our systematic research on the direct CVD growth of graphene on mechanically exfoliated hBN flakes. With our newly discovered gaseous surface activator, silane can effectively work with C₂H₂, allowing for the synthesis of precisely aligned single crystalline graphene on hBN. The growth rate reaches 1 um/min, a 2-3 orders of magnitude increases compared to previous reports. Typical domain sizes are 10-20 microns, already in a range for practical device demonstration. We will present our results on the synthesis of single crystalline hBN domains over hundred micrometers in size, using an optimized Cu-Ni alloy as catalyst. Results on vertical and lateral graphene/hBN hetero-structures made on CVD hBN will be discussed. Most recent progresses on the fabrication of inch-sized graphene wafer from a controlled single nucleus and the synthesis of A-B stacked double layer graphene with large domain sizes will also be briefly presented.

Topological insulator with magnetic order is expected to host rich physics. In this presentation, we present the proximity effect between topological insulators Bi₂Se₃ thin films and magnetic insulators BaFe₁₂O₁₉ with out-of plane magnetocrystalline anisotropy and Y₃Fe₅O₁₂ with in-plane magnetocrystalline anisotropy. Electron transport measurements reveal the parabolic positive magnetoresistance (MR) in perpendicular fields and negative MR in parallel fields in Bi₂Se₃/ BaFe₁₂O₁₉ heterostructure, suggesting the strong magnetic exchange interaction. In Bi₂Se₃/ Y₃Fe₅O₁₂, we report magnetization-related negative in-plane MR and the suppression of weak antilocalization effect, which may be attributed to the quantum domain wall resistance. This work gives us a deeper understanding of complex magnetic interface effects.

In this talk, I'll introduce the off-diagonal Bethe Ansatz method, a newly developed analytic theory for solving the quantum integrable models. With the XYZ model and the spin torus model as concret examples, I show how this theory works for the integrable but previously "unsolvable" models. The methods to derive the spectrum and to retrieve the eigenvectors and scalar product will be included.

Weyl and Dirac semimetals recently stimulate intense research activities due to their novel properties. Combining first-
principles calculations and effective model analysis, we predict that nonmagnetic compounds BaYBi (Y=Au, Ag and Cu)
are Dirac semimetals. As for the magnetic compound EuYBi, although the time reversal symmetry is broken, their long-range
magnetic ordering cannot split the Dirac point into pairs of Weyl points. However, we propose that partially substitute Eu ions
by Ba ions will realize the Weyl semimetal.

The proximity properties of edge currents at the interface between the normal and superconducting graphene in the presence of magnetic field are investigated. At first, the edge states introduced by Andreev reflection at the graphene/superconductor interface is discussed with zigzag and armchair boundary respectively. Then the Hall edge current along the direction parallel to the interface is investigated for current density distributions and total current along the zigzag and armchair boundary respectively.

The recently discovered superconductor K2Cr3As3 involves six Cr atoms in a unit cell and exhibits Luttinger liquid behavior in its normal state, representing a prototype of molecular orbital superconductors with electron correlations and reduced dimensionality. We solve the molecular-orbital bands in a twisted Hubbard tube modelling the CrAs structure in this compound, by assuming the quasi-degenerate atomic orbitals. We then study the low energy property of the three active molecular-orbital bands using the deduced three-channel Tomonaga-Luttinger Hamiltonian. The implications for the possible unconventional spin triplet superconductivity in K2Cr3As3 are discussed.

Based on first-principles calculations, we systematically investigated the topological surface states of Bi and Sb thin films of 1–5 bilayers in (111) orientation without and with H(F) adsorption, respectively.
We find that compared with clean Bi and Sb films, a huge band gap advantageous to observe the quantum spin Hall effect can be opened in chemically decorated bilayer Bi and Sb films, and the quantum phase transition from trivial (non-trivial) to non-trivial (trivial) phase is induced for a three bilayer Bi film and single (four) bilayer Sb film. Surface adsorption is an effective tool to manipulate the geometry, electronic structures and topological properties of film materials.

A beam splitter consisting of two normal leads coupled to one end of a topological superconducting nanowire via double quantum dot is investigated. In this geometry, the linear current cross-correlations at zero temperature change signs versus the overlap between the two Majorana bound states hosted by the nanowire. Under symmetric bias voltages the net current flowing through the nanowire is noiseless. These two features highlight the fermionic nature of such exotic Majorana excitations though they are based on the superconductivity. Moreover, there exists a unique local particle-hole symmetry inherited from the self-Hermitian property of Majorana bound states, which is apparently scarce in other systems. We show that such particular symmetry can be revealed through measuring the currents under complementary bias voltages.

We theoretically investigate the electronic properties of Dirac semimetal nanostructures. Quantum confinement generally opens a bulk band gap at the Dirac points. We find that confinement along different directions shows strong anisotropic effects. In particular, the gap due to confinement along vertical c-axis shows a periodic modulation, which is absent for confinement along horizontal directions. We demonstrate that the topological surface states could be
controlled by lateral electrostatic gating. It is possible to generate Rashba-like spin splitting for the surface states and to shift them relative to the confinement-induced bulk gap. These results will not only facilitate our fundamental understanding of Dirac semimetal nanostructures, but also provide useful guidance for designing all-electrical topological spintronics devices.

Non-centrosymmetric transition metal monopnictides are promising Weyl semimetals (WSMs) with exotic physical properties. Although chiral WSM states have been observed in TaAs and NbAs, there is no conclusive evidence on the existence of Weyl fermions in NbP. Here, we use angle dependent quantum oscillations to reveal that NbP has four pairs of unusually large Weyl fermion pockets in the kz=0 plane near the high symmetry points Sigma, dominating over the coexisting massive hole pockets and the previous reported WSM pockets in the kz=1.18pi/c plane. Such dominant WSM pockets are highly anisotropic in k-space and approaching the parabolic band top along the internode direction. The corresponding Fermi surface is consisting of helical Weyl fermions with unprecedented mobility of 1*10E7 cm2V-1s-1 at 1.5 K, well protected from defect backscattering by real spin conservation associated to the chiral Weyl nodes. Inter-pocket pumping of Weyl fermions with opposite helicity becomes feasible when the magnetic field and electric field are applied in parallel, manifested as robust chiral anomaly induced negative MR, another quantum signature of WSMs.

Motivated by recent discovery of strongly spin-orbit coupled two-dimensional (2D) electron gas near the surface of Rashba semiconductors BiTeX (X=Cl, Br, I), we calculate thermoelectric responses of spin polarization in 2D Rashba model using an exact solution of the linearized Boltzmann equation for elastic scattering. When the Fermi energy $E_{F}$ lies below the band crossing point we find a non-Edelstein electric-field induced spin polarization which is linear in $E_{F}$. We show that the Mott-like relation between spin polarizations induced by the temperature gradient and electric field breaks down significantly when $E_{F}$ lies in the vicinity of the band crossing point. As the temperature tends to zero, the temperature-gradient induced spin polarization vanishes. These results differ from previous ones obtained by relaxation time approximations.

Theoretical and experimental study on the thermoelectric materials

Xun Shi*, Wenqing Zhang, and Lidong Chen

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

 

Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation and offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat.  The key to the development of advanced TE technologies is to find highly efficient TE materials. In current commercial materials, the zTs are limited to values around unity. Recently, several novel concepts have been proposed to enhance the efficiency of TE materials and laboratory results suggest that high zT values can be realized in several families of bulk materials.  In this presentation, we show the theoretical and experimental study on the thermoelectric properties of a few copper-based compounds. We will show these materials possess abnormal thermoelectric properties with low thermal conductivity and good thermoelectric figure of merit.  The physical mechanisms behind these abnormal thermoelectric properties will also be discussed to show the possibility of realization of ultrahigh thermoelectric figure of merit.

Corresponding author*: xshi@mail.sic.ac.cn

Gauge fields and non-Abelian gauge fields in graphene

Lin He (何林)

 

The Center of Advanced Quantum Studies, Department of Physics, Beijing Normal University, Beijing, 100875, P. R. China

（高等量子研究中心，物理学系，北京师范大学，北京，100875）

 

In this talk, I will introduce several types of (non-Abelian) gauge fields in graphene and show their effects on electronic properties of graphene. With the help of the gauge fields generated by magnetic fields, we realize Landau quantization in graphene monolayer, Bernal bilayer and trilayer. In a gapped graphene monolayer, we direct image the two-component Dirac-Landau levels in atomic resolution. In strained graphene, lattice deformations induce valley-dependent gauge (pseudomagnetic) fields. I will show direct and compelling evidence for electron confinement and valley splitting in grapheme monolayer due to the coexistence of both the magnetic field and the pseudomagnetic field.

Non-Abelian gauge potentials are quite relevant in subatomic physics, but they are relatively rare in a condensed matter context. In this talk, I will also show the experimental evidence for non-Abelian gauge potentials in twisted graphene bilayers. We direct demonstrate that the non-Abelian gauge potentials in twisted graphene bilayers confine low-energy electrons into a triangular array of quantum dots following the modulation of the Moiré patterns.

 

Electronic devices of two-dimensional semiconductors

- from atomic to molecular

Xinran Wang
School of Electronic Science and Engineering, Nanjing University
xrwang@nju.edu.cn

Two-dimensional materials (including graphene, MoS₂, etc.) represent a promising class of materials for electronic and photonic devices, benefiting from their unique properties such as extremely high mobility and ultrathin body. In this talk I will present our works on electronic devices based on 2D atomic and molecular semiconducting crystals.

In the first part, we provide direct evidence that sulfur vacancies exist in exfoliated MoS₂, introducing localized midgap donor states. At low carrier density, the charge transport in MoS₂ is by electron hopping through these localized states, leading to much lower mobility than theoretical expectations and insulating behavior. We develop a facile low-temperature thiol chemistry to repair the sulfur vacancies and improve the interface, resulting in significant reduction of the charged impurities and traps in MoS₂. High mobility greater than 80cm² V^-1 s^-1 is achieved in backgated monolayer MoS₂ field-effect transistors for the first time. We further develop a theoretical model to quantitatively extract the key microscopic quantities that control the transistor performances, including the density of charged impurities, short-range defects and traps.

In the second part, we demonstrate that high-quality few-layer dioctylbenzothienobenzothiophene molecular crystals can be grown on graphene or boron nitride substrate via van der Waals epitaxy, with precisely controlled thickness down to monolayer, large-area single crystal, low process temperature and patterning capability. As a result of the pristine crystal and interface quality, monolayer dioctylbenzothienobenzothiophene molecular crystal field-effect transistors on boron nitride show record-high carrier mobility up to 10cm²V^-1s^-1. Our work unveils an exciting new class of two-dimensional molecular materials for electronic and optoelectronic applications.

 

References

1. Hao Qiu, Tao Xu, Zilu Wang, Wei Ren, Haiyan Nan, Zhenhua Ni, Qian Chen, Shijun Yuan, Feng Miao, Fengqi Song, Gen Long, Yi Shi, Litao Sun, Jinlan Wang* & Xinran Wang*, Nature Comm. 4, 2642 (2013).
2. Hao Qiu, Lijia Pan, Zongni Yao, Junjie Li, Yi Shi* and Xinran Wang*, Appl. Phys. Lett., 100, 123104 (2012).
3. Zhihao Yu, Yiming Pan, Yuting Shen, Zilu Wang, Zhun-Yong Ong, Tao Xu, Run Xin, Lijia Pan, Baigeng Wang, Litao Sun*, Jinlan Wang, Gang Zhang, Yong Wei Zhang, Yi Shi* & Xinran Wang*, Nature Comm. 5, 5290 (2014).
4. Daowei He, Yuhan Zhang, Qisheng Wu, Rui Xu, Haiyan Nan, Junfang Liu, Jianjun Yao, Zilu Wang, Shijun Yuan, Yun Li, Yi Shi*, Jinlan Wang*, Zhenhua Ni, Lin He, Feng Miao, Fengqi Song, Hangxun Xu, K. Watanabe, T. Taniguchi, Jian-Bin Xu, and Xinran Wang*. Nature Comm. 5, 5162 (2014).

 

 

Effects of external fields on electronic properties of silicene

Jun Ni

Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University, Beijing 100084.

Collaborative Innovation Center of Quantum Matter, Beijing 100084.
Email: junni@mail.tsinghua.edu.cn

 

Silicene nanostructures have both rich electronic and magnetic properties, and thus have potential applications for silicon-based electronic and spintronic nano-devices.  In this talk, I will show our results of the effects of thermal and electric fields on the electronic properties of silicene nanostructures based on first-principles calculations and statistical analysis of canonical ensembles. The effects of electric field on the energy gap and the edge bands near the Fermi level of silicene nanoribbons are discussed. We find that the electric field can induce the gapless spin semiconductor of silicene nanoribbons. The electric field induced features can be achieved by a suitable uniaxial compressive strain. This can be understood from the effect of the Wilson transition. I will also discuss the magnetic properties of the silicene doped with Fe and Cr metal atoms under external electric field. We find that the doped systems show a variety of interesting magnetoelectric behaviors. The magnetic moment of Fe doped silicene show a sharp jump at a threshold electric field, which indicates a good switching effect. The magnetic moment of the doped systems has a nearly linear region with the electric field. We find that the changes of magnetic moment strongly depend on the direction of the electric field. In particular, one configuration of Fe doped silicene shows an interesting magetoelectric response which can be considered as a magetoelectric diode.  At last, I will show the effects of thermal fields on the electronic properties of silicone and come to the following conclusions: i) The band structure features near the Fermi energy in the pristine silicene are protected in thermal field, which means that the high transferability of the Dirac electrons is retained. ii) The modulation of the electronic properties of silicene in a thermal field needs much smaller electric field than the silicene without thermal field. The higher temperature corresponds to a larger gap under the same electric field.

 

 

On the synthesis of hBN, graphene and graphene/hBN hetero-structures by chemical vapor deposition

Tianru Wu¹, Shujie Tang¹, Guanyuan Lu¹, Haomin Wang¹, Qinghong Yuan², Feng Ding³, Qingkai Yu⁴, Xiaoming Xie^1,5

 

¹ State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, P.R. China

² Department of physics, East China Normal University, 500 Dongchuan Road, Minhang, Shanghai 200041, China

³ Institute of Textiles and Clothing, Hong Kong Polytechnic University, Kowloon, Hong Kong, 999077, P. R. China

⁴Ingram School of Engineering, and MSEC, Texas State University, San Marcos, Texas, 78666, USA

⁵ School of Physical Science and Technology, ShanghaiTech University, 319 Yueyang Road, Shanghai 200031, China

Hexagonal　boron nitride (h-BN), as a dielectric substrate, can largely preserve the superior properties of graphene. Vertical or lateral graphene/hBN hetero-structures may bring in opportunities for novel electronic devices. Scalable synthesis of high quality single crystalline hBN, graphene and graphene/hBN are strongly desired, but remains to be extremely challenging. In this talk, I will present our systematic research on the direct CVD growth of graphene on mechanically exfoliated hBN flakes. With our newly discovered gaseous surface activator, silane can effectively work with C₂H₂, allowing for the synthesis of precisely aligned single crystalline graphene on hBN. The growth rate reaches 1 um/min, a 2-3 orders of magnitude increases compared to previous reports. Typical domain sizes are 10-20 microns, already in a range for practical device demonstration. We will present our results on the synthesis of single crystalline hBN domains over hundred micrometers in size, using an optimized Cu-Ni alloy as catalyst. Results on vertical and lateral graphene/hBN hetero-structures made on CVD hBN will be discussed. Most recent progresses on the fabrication of inch-sized graphene wafer from a controlled single nucleus and the synthesis of A-B stacked double layer graphene with large domain sizes will also be briefly presented.

Electronic Transport and Device Applications of 2D Materials

 

Feng Miao (缪峰)

Nanjing University
 

 

During the last decade, tremendous research efforts have been focused on two-dimensional (2D) materials due to their rich physics and great potentials for many applications. The first part of my talk will focus on the electro-mechanical properties of suspended graphene, which is the thinnest flexible conductive material. I will present the direct observation and controlled creation of one- and two-dimensional periodic ripples in suspended graphene sheets, using both spontaneously and thermally generated strains.We are able to control ripple orientation, wavelength and amplitude by making use of graphene’s negative thermal expansion coefficient and electrostatic force. The latest results on the layer-number dependent positive piezoconductive effect will also be discussed [1].

The second part of the talk will cover our recent studies on atomically thin rhenium disulfide (ReS₂) flakes with unique distorted 1T structure[2],  which exhibit interesting in-plane anisotropic transport and mechanical properties, as well as excellent optoelectronic properties. We fabricated mono- and few-layer ReS₂ field effect transistors, which exhibit competitive performance with large current on/off ratios (~10⁷) and low subthreshold swings (100 mV dec^-1). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known 2D semiconducting materials. We further successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS₂ anisotropic field effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits [3]. Our latest results on the ultra-high photoresponsivity phototransistors based on few-layer ReS₂ with successful demonstration of weak signal detection will also be presented [4].

 

[1] Xu, et al. “The positive piezoconductive effect in graphene”, (2015) (under review)

[2] Feng, Zhou, et al. “Raman vibrational spectra of bulk to monolayer ReS₂ with lower symmetry”, (2015) (under review)

[3] Liu, Fu, et al. “Integrated Digital Inverters Based on Two-dimensional Anisotropic ReS₂ Field-effect Transistors”, Nat. Comm.6, 6991 (2015)

[4] Liu, et al. “Ultra-high responsivity phototransistors based on few-layer ReS₂ for weak signal detection”, (2015) (under review)

        石墨烯具有巨大的比表面积以及优异的载流子迁移率和导电能力，同时还拥有良好的机械负载能力和吸附能力，这些优异的性能特征使基于石墨烯的功能复合材料在光催化制氢和光催化降解等多个领域都展示出巨大的应用潜力。以石墨烯—二氧化钛复合结构为例，作为支撑体的石墨烯不仅可以很好的承载和分散二氧化钛颗粒，而且石墨烯优越的电荷传输能力还可以将二氧化钛表面存在的光生电荷很快转移，从而增加光生电子空穴的分离速率，降低其复合概率，大幅提高光电转换能力。然而，这种简单的二元复合结构对光波的利用仍局限于紫外光和极有限的蓝紫光，对长波段特别是红外光的利用相当匮乏。

        我们提出通过利用多元共混材料的功能协同来增强光电转换材料效率的思路，即利用上转换材料吸收太阳光中的高频部分，转换并发射出高能量的紫外光，拓展复合材料的光谱吸收范围；同时借助石墨烯促进光生电子-空穴对的分离，实现二氧化钛光催化效率的提高。我们采用简单的水热处理方法，成功的制备出由石墨烯、P25(TiO2)纳米颗粒和上转换纳米晶组成的三元石墨烯共混复合材料，测试其光电化学性能发现，该三元复合材料在太阳光照射下具有良好的光电转换性能。

氮化物半导体异质结构材料具有宽的直接带隙，优异的物理、化学稳定性，高饱和电子漂移速度，高击穿场强等优越性能，是发展高频、高温、高功率电子器件的优选材料。以电子自旋自由度为基础的自旋电子学器件以速度快、功耗低、集成度高等优点，在未来的信息技术领域有着广泛的应用前景。氮化物半导体具有较长的自旋驰豫时间和高于室温的居里转变温度。同时具有很强的自发和压电极化电场，导致了较强的自旋轨道耦合。因此，开展对氮化物半导体异质结构材料自旋性质的研究对丰富和发展半导体低维物理和半导体自旋电子学以及器件应用有重要的科学意义。

圆偏振光激发产生的自旋极化电子在电场的驱动下，由于反常霍尔效应的机制发生横向偏转而形成横向电流，这一现象被称为光致反常霍尔效应（PIAHE）。反常霍尔效应最早的观察是在铁磁性材料中，之后随着自旋霍尔效应得到广泛关注。利用Rashba自旋轨道耦合较强的Al_0.25Ga_0.75N/GaN异质结构在室温下通过光注入自旋极化的方法观测到反常霍尔电流随着纵向电场增大而线性增大。认为本征的反常霍尔机制对观测到的反常霍尔电流有贡献。利用圆偏振光引起的涡旋电流，在InN薄膜中观测到了逆自旋霍尔效应(RSHE)。发现InN薄膜的RSHE电流是表面电子积累层和体部分的共同贡献。研究了InN的极性对RSHE电流的影响，发现在In极性InN中，表面电子积累层中的自旋轨道耦合系数与体中的系数是反号的，而在N极性两者的符号是相同的。提出一种基于PIAHE和RSHE研究自旋输运的方法。在施加纵向电场前后，分别测得RSHE电流和存在电场时总自旋光电流随着光斑位置的变化曲线，通过分析总自旋光电流出现极值的光斑位置，确定Al_0.25Ga_0.75N/GaN异质结构中2DEG的自旋扩散系数和自旋霍尔迁移率的比值。

自2012年钙钛矿太阳能电池的概念被首次提出后，钙钛矿电池因为制造工艺简单、成本低廉、效率高、对环境友好等优势而得到广泛的关注。在短短两年多，钙钛矿电池的光电转换效率已经超过20%，远远超过第三代有机太阳能电池和无定型硅电池，接近市场准入门槛。然而，一些问题仍未解决，限制了效率的进一步提高，如钙钛矿电池的回滞现象，即电池从正向偏压扫到短路电流和反过来时有不同光电流-光电压表现。从原子尺度探索钙钛矿电池奇异的回滞现象起因，有助于人们加深对器件工作机理的认识，进一步优化钙钛矿电池，推动其大规模市场化生产。

基于密度泛函理论，我们用第一性原理计算模拟了二氧化钛和钙钛矿界面的电荷传输过程，发现电子从二氧化钛注入到钙钛矿的时间超快（约4 fs），而电子复合的时间则超慢（约46 ps）。同时我们还研究了氧缺陷和钙钛矿中MA有机基团的取向对界面电荷传输的影响。通过在在二氧化钛表面、中间层和底层引入氧缺陷，我们发现氧缺陷会在费米能级下引入一个占据的缺陷态。这个缺陷态对电荷的注入基本没有影响，但是却会加快电子复合过程。并且不同位置的氧缺陷所对应的复合寿命也不同。由于这些缺陷态之间的能量差只有几十个至几百个meV，所以我们推测在测量中的电压使得TiO2上的氧缺陷发生径向移动，从而导致不同的光电流-光电压输出。

本文的工作建立了微观界面结构和宏观器件表现之间的联系，提供了钙钛矿太阳能电池领域器件效率优化的新思路。

The development of advanced energy storage devices is at the forefront of research geared towards a sustainable future. Nanostructured materials are advantageous in offering huge surface to volume ratios, favorable transport features, and attractive physicochemical properties. They have been extensively explored in various fields of energy storage and conversion. The abundance and low cost of the Na resource will become attractive when a large amount of electrode materials are demanded for electrochemical energy storage applications. As an alternative, Na-ion batteries have recently drawn significant attention because of lower cost and larger abundance of Na resources. In this work, strongly coupled graphene-TiO₂ hybrid displays excellent rate capability and cyclability. The diffusion channels at the interface of strongly coupled graphene-TiO₂ on the nanoscale offer low diffusion barrier pathways for sodium intercalation/deintercalation, effectively serving as nanohighways for sodium transport. Kinetics analysis reveals an interesting Na⁺ intercalation pseudocapacitive behavior in the G-TiO₂ sodium cell and a high contribution of capacitive charge. Our findings will hold the great promise for the development of long-life Na-ion batteries for next-generation large-scale energy storage applications.

 开路电压是衡量锂离子电池性能的重要参数，其在外电路的表现为电势，而在内电路表现为电势与化学势的共同效应。在锂离子电池中，各种界面的电阻通常很大，界面处的电势与化学势的降落尤其显著。报告主要以偏铝酸锂的计算为例，关注电极与电解质界面中，电势与化学势的变化情况，以及其对界面处锂离子微观输运机制的影响。计算结果表明，电势与化学势的变化所引起的非电中性与非化学计量比，可以显著提高锂离子电导率，并解释低电导率包覆材料并不会影响锂离子电池性能的原因。该方法与结论具有普适性，除应用于包覆，同样可以应用于低电导率无机固体电解质界面相（Solid Electrolyte Interphase)，以及电解质和界面的缓冲层；报告综述并重新解释相关的实验现象。

Solid-state thermoelectric technology uses electrons or holes as the working fluid for heat pumping and power generation and offers the prospect for novel thermal-to-electrical energy conversion technology that could lead to significant energy savings by generating electricity from waste industrial heat.  The key to the development of advanced TE technologies is to find highly efficient TE materials. IRecently, several novel concepts have been proposed to enhance the efficiency of TE materials and laboratory results suggest that high zT values can be realized in several families of bulk materials.  In this presentation, we show the study on the thermoelectric properties of Cu_2-δX-based (X=S, Se, and Te) materials. We will show these materials possess interesting thermoelectric properties with ultralow thermal conductivity and good thermoelectric figure of merit.  The physical mechanisms behind these abnormal thermoelectric properties will also be discussed. Finally, the stability of these compounds will also be presented and discussed.

电子产品的日益小型化，功能化、集成化，以及电动汽车和可再生能源发电并网的调峰储能，人们对提供能源的二次电池提出了更高的要求。需要电池具有更小的尺寸、更轻的重量及更高的性能。因此，新型高比容量、高倍率性能的锂离子电池电极材料的开发极具迫切性。高性能的电极材料的研发制备技术是有待突破的关键技术之一。

另外，室温钠离子电池与锂离子电池具有相似的储能机制，但钠的资源丰富，原料成本低廉，对于可再生能源的大规模储能和智能电网来说室温钠离子电池表现出极大潜力。然而由于Na⁺半径相比Li⁺大很多，Na⁺反复的嵌入/脱出极易导致多次循环后电极的结构陷，从而引起容量的衰减。因此，探索合适的室温钠离子电池的正、负极材料是急需解决的关键问题。

本报告中，我们将重点介绍我们通过微纳结构的设计、调控实现提高锂（钠）离子电池性能的目的。在此领域我们开展了一系列研究,重点包括MoS₂,Ge；Li₄Ti₅O₁₂;FeS₂ 等。^[1-4] 在锂电池电极材料方面，我们通过构筑新型纳米多孔、纳米交联网络和纳米阵列等新型三维纳米结构集流体材料，缓解了体积膨胀；独特的三维多孔集流体设计有效的减小电子（e^-）传输距离，为体积膨胀提供了缓冲的空间，并且有利于电解液浸润，使得电极材料的倍率性能得到大幅度的提高。

除此之外，在钠离子电池正极材料方面，我们通过构筑一维、核/壳结构的Na₃V₂(PO₄)_3，提高了材料的电导率，缓解了材料在充放电过程中的内部应力，实现了高倍率、长循环寿命的Na₃V₂(PO₄)₃的正极材料。^[5]

参考文献：

1. C. Zhu, X. Mu, P. A. Vanaken, Y. Yu* and J. Maier*, Angewandte Chemie - International Edition, 2014, 53, 2152-2156.

2.J. Liu, Y. Wen, Y. Wang, P. A. van Aken, J. Maier and Y. Yu*, Advanced Materials, 2014, 26, 6025-6030.

3.J. Liu, K. Song, P. A. Van Aken, J. Maier and Y. Yu*, Nano Letters, 2014, 14, 2597-2603.

4. J. Liu, K. P. Song, C. Zhu, C.-C. Chen, P. A. van Aken, J. Maier , Y. Yu*, ACS Nano,2014, 8 (7), 7051–7059.

5. C. Zhu, K. Song, P. A. Van Aken, J. Maier and Y. Yu*, Nano Letters, 2014, 14, 2175-2180.

In recent years, there has been remarkably rapid progress in the development of high brightness light-emitting diodes (LEDs) based on InGaN/GaN multiple quantum wells (MQWs). However, to attain the long-term performance targets, great efforts will need to achieve high emission efficiency. Among many recent approaches to the improvement of the inefficiency, localized surface plasmon (LSP) enhanced emission has been actively studied. Surface plasmon study has become one of the most attractive topics in the field of nanotechnology. Surface plasmon can be understood as the free electron collective movement alone metal surface driven by electric field component of an incident electromagnetic wave. When the surface plasmon resonance with light waves, the light waves will be limited in the range of tens of nanometers around metal surface and the electromagnetic field is greatly enhanced.

In this report, surface plasmon enhanced LEDs with nanoporous surface structures were fabricated, and the mechanisms of emission enhancement were studied. The LSP enhanced blue InGaN-based LEDs with metal nanoparticles embedded in the nanopores of the p-GaN surface were fabricated using Ni nanoporous templates. The optical output power of blue LEDs were enhanced more than 40% compared to the conventional LEDs. Through a comprehensive analysis of EL spectrum, consumer spectrum, the gain spectrum and different coupling distance, we have studied the coupling mechanism. It is found that the emission enhancements were highly dependent on the LSP coupling distance between metal particles and MQWs, the maximum emission enhancement increased and shifted to LSP resonance energy with the distance decreased from 60nm to 30nm.

硅烯是单元子层的蜂窝状结构的硅，具有类似石墨烯，但能隙更大、且更容易实现电场调控的Dirac电子态。近年来硅烯领域迅速发展，包括其制备、电子态、器件原型均得到了报道。但硅烯在空气中易于氧化，其应用受到制约。为此对硅烯进行改性，合成基于硅烯的更稳定的二维材料，是将来可能的应用途径。而且理论研究表明氢化或卤化有可能保留硅烯的特殊电子态，并引入磁性等有趣性质。本报告研究了硅烯的氢化和卤化，在两种情况下均获得了二维有序结构，并对其吸附机理进行了深入研究和理解。

石榴石型锂离子导体（Li₇La₃Zr₂O₁₂，LLZO）具有高的室温离子电导率、对锂稳定和电化学窗口宽等优点，是目前十分受人关注的制备固态二次锂电池的固体电解质材料。为了获得高电导率的LLZO材料，稳定立方相并有效控制锂含量是其中的关键问题。工作中，我们探索了异质元素掺杂、液相烧结、气氛烧结以及热压烧结等多种技术手段，掌握了制备高电导率以及高致密陶瓷LLZO的关键工艺，最终获得相对致密度99%和室温离子电导率大于10^-3 S/cm的陶瓷电解质。

利用所获得的致密的高电导LLZO陶瓷，加工制备了厚度为50微米到1毫米的陶瓷片，将它应用于固态锂离子电池和固态锂空气电池，探讨了可行性及其中的关键科学问题。发现以LLZO陶瓷片作为电解质和支撑体，以LiFePO₄作为正极材料和Li为负极材料的固态电池可获得130 mAh/g的放电容量，循环100次具有90%以上的容量保持率。更有意思的是，以LLZO陶瓷片作为支撑体的固态锂空气电池，可完全在开放的空气中多次循环，其中的机理是Li₂CO₃在放电和充电过程中的形成和分解。这种新型全固态锂空气电池克服了有机电解液锂空气电池不能在空气中工作的瓶颈，具有广阔的应用前景。

       锂离子固体电解质是指锂离子电导率接近(或在有些情况下超过)熔盐和电解质溶液的一类固体材料。它所指的是一种具有液-固二象性的特殊状态，其中的锂离子具有接近液体的迁移率，而其它离子则保持规则的结晶排列,这种液-固二象性，正在吸引着凝聚态物理学家们的注意。

     典型的固体电解质材料作为具有新奇现象的独立相，一般具有有如下特征：离子电导率高；激活能低；晶体为敞形结构(open structure)，这种敞形结构具有一种离子可以占据的相互连接的空位置网络。此外，快离子导体具有重要的动态效应和集体效应：它没有清晰的光学晶格模，但存在着弥散的低能激发，与频率有关的电导率红外峰，异常的核磁共振前因子，相变，以及在两个容许位置之间发现运动离子的高度可能性。从基础研究考虑，固体电解质的核心问题是离子在体相和表面、界面的输运。本报告将围绕若干种典型的固体电解质材料中的输运物理问题研究历史进行介绍，并着重结合本人和合作伙伴开展的一些工作介绍近些年来在这方面的主要进展，并对今后计算物理学/材料学在固体电解质材料研究中扮演的角色进行了展望。

GaN基蓝光LED是半导体照明技术的核心器件，其效率直接决定了白光LED的光效。尽管其量子效率已经很高，但发光机理还存在许多尚未研究清楚的地方。本文以作者从事相关研究的结果为基础，总结归纳了目前学术界针对InGaN量子阱LED中载流子输运和复合的研究，重点是InGaN中的载流子局域化效应和LED中的量子效率下降效应，并介绍了相关的效率评测方法。

Half-Heusler compounds are important high temperature thermoelectric materials having attracted considerable attention in the recent years. However, developing high performance p-type HH compounds with zT>1 has been a big challenge. In this talk, we show that a new p-type HH alloy with a high band degeneracy, Ti doped FeV_0.6Nb_0.4Sb, can achieve a high zT of 0.8. Increasing Nb content in the Fe(V_1-yNb_y)Sb solid solutions can achieve lower valence band effective mass and consequently higher carrier mobility. Furthermore, increasing Nb content in Fe(V_1-yNb_y)_1-xTi_xSb will also prevent the degradation of the TE performance at high temperatures due to the increased band gap. Thus we obtain a high zT of 1.1 at 1100K for FeNb_1-xTi_xSb without V substitution, which is the highest reported value for p-type HH compounds. In view of abundantly available elements, good stability and high zT, FeNbSb based alloys can be great promising for high temperature power generation.

氢气是重要的清洁能源,电解水制氢是最有效的可持续方法。虽然传统贵金属催化剂(例如:铂金)具有较高的催化效率,但严重受限于其自然稀缺性和高昂成本,因此亟需开发经济、稳定、高效的新型催化剂。二硫化钼具有于Pt类似的氢原子结合能, 具有催化活性高,成本低廉等优点，是贵金属电催化剂的重要替代品。最近有文献用密度泛函理论计算揭示出氢原子在硒化钼边缘的吸附率优于硫化钼，这说明硒化钼也是一种潜在的电催化制氢材料，甚至性能可能更加优异。本文采用水热插层剥离法成功制备硒化钼纳米片，制备的硒化钼纳米片具有比块体多的吸氢活性位点，具有很好的催化活性。除了优异的催化材料外，电极的构建也是电化学制氢的关键，电泳沉积（Electrophoretic deposition, EPD）不仅可以对各种外形和复杂多孔的结构进行涂敷，还可以用于纳米器件的连接与组装。本文利用电泳沉积，将制备的硒化钼纳米片沉积到各种导电基板上，构成析氢电极。通过一系列的电化学测试和分析，对硒化钼纳米片的析氢性能进行了系统的研究，同时发现通过电泳沉积到三维碳纤维上的硒化钼具有最为优异的析氢性能。

        水热硫化法生长单晶Co₃S₄纳米片介孔材料在泡沫镍上，首先水热法在泡沫镍上合成氢氧化钴碳酸盐，退火转换为黑色晶石Co₃O₄纳米线结构, Co₃O₄纳米线阵列作为牺牲模板在Na₂S溶液中水热法转化为Co₃S₄纳米片。本文运用X-射线衍射、SEM和电化学工作站对的材料的结构和电化学电容性能进行了研究。不同阶段得到产物的XRD衍射结果可以看到，最终水热硫化Co₃O₄转化为Co₃S₄。SEM表征Co₃S₄纳米片阵列的形貌，可以看到纳米片垂直生长在衬底上并且互相交联。用电化学工作站测试在同一电流密度24mA/cm²，Co₃O₄纳米线和Co₃S₄纳米片3000次循环充放电后分别保留84.1%和96.2%的电容，显然Co₃S₄纳米片电极展示了更高的电容和更好的充放电循环性能。

Effective spin mixing conductance of ferromagnetic insulator-metal interfaces and magnetic tunnel junctions

 

Huiming Tang, Lei Wang, Ke Xia

 

Department of Physics, Beijing Normal University, Beijing 100875, China

 

akexia@bnu.edu.cn

 

Normal metal(N) interfaces is the so-called spin mixing conductance that governs the absorbance of a transverse spin current and generation of spin transfer torque. The spin-mixing conductance is a purely non-relativistic concept, while there is mounting evidence that spin-orbit interaction at interfaces is important(such as spin-flip at interface, interface spin-orbit torques). Here we address the effective spin mixing conductance for the metal–ferromagnetic insulator interfaces and magnetic tunnel junctions by first principles calculations. We predict an effective spin mixing conductance for metal–ferromagnetic insulator interfaces that are anisotropic in the presence of spin-orbit coupling, which indicates that the spin pumping and its relate spin transfer torques can be turned with spin orbit interaction.

 

新型二维半导体光电材料与器件

李京波

中国科学院半导体研究所，北京市海淀区清华东路甲35号，100083

以单层过渡金属硫化物(transition metal dichalcogenides)为主的二维半导体是继石墨烯之后最受关注的新型光电材料，给未来信息和能源领域带来技术性的革命。其二维特性也提供了一个研究低维系统中电子、光子、和声子行为的完美平台。最近我们在新型二维半导体系统研究中,(1)发现二维单层MoS₂/MoSe₂的异质结形成莫氏图案的成因(Nano Lett.,13,5485,2013)^[1];(2)发现多层MoSe₂中的热致间接-直接带隙的转换(Nano Lett.12,5576,2013)^[2];(3)发现并深刻阐明了二维半导体的点缺陷和激子的相互作用(Nano Lett. 13, 2831,2013; Sci. Rep.3, 2657,12013) ^[3,4];(4)发现硅烯纳米带中的自旋滤波和磁阻效应^[5];(5)在ReS₂二维材料的研究中,发现单层与体材料电子结构一致(Nature Comm.,5,3252, 2014)^[6];(6)二维MoS₂/WS₂异质结表现大幅度提高的场效应开关比(Adv. Func. Mater., 24, 7025, 2014)^[7]；（7）在超薄二维GaS半导体红外光探测器中发现很高的外部量子效应(Nanoscale, 6, 2582, 2014)^[8]；（8）通过应力改变ReSe₂二维材料的电子结构和光学性质（Nano Letters, 15, 1660, 2015）^[9]；（9）在国际上首次获得大尺寸Co_0.16Mo_0.84S₂ 双层合金及其场效应研究（ACS Nano, 9, 1257, 2015）^[10]；等等。这些研究为将来理解二维半导体光电材料新奇物理现象打下坚实基础。同时,这些重要的物理现象意味着未来新型二维微纳光电器件与相关工艺将会出现新的机遇与挑战。

【参考文献】

[1] J. Kang, Jingbo Li, S. S. Li, J.B. Xia, and L-W. Wang, Nano Letters, 13, 5485 (2013).

[2] S. Tongay, J.Zhou, C. Ataca, K. Lo, Jingbo Li, J. C. Grossman, and J. Wu*, Nano Letters 12, 5576 (2012).

[3] S. Tongay, J. Zhou, C. Ataca, J. Liu, J. S. Kang, T. S. Matthews, L. You, Jingbo Li, J. C. Grossman, and J. Wu*, Nano Letters, 13, 2831(2013).

[4] S. Tongay, J. Suh, C. Ataca, W. Fan, A. Luce, J. S. Kang, J. Liu, C. Ko, R. Raghunathanan, J. Zhou, F. Ogletree, Jingbo Li, J. C. Grossman, and J. Wu*, Scientific Report, 3, 2657 (2013).

[5] J. Kang, F. Wu, and Jingbo Li*, Appl. Phys. Lett. 100, 233122 (2012).

[6] S. Tongay, H. Sahin, C. Ko, A. Luce, W. Fan, J. Zhou, Y. S. Huang, J. Yan, F. Ogletree, S. S. Li, Jingbo Li, F. M. Peeters, and J. Wu*, Nature Comm., 5，3252 (2014).

[7] N. Huo,J. Kang, Z. Wei, S.-S. Li, Jingbo Li*, S.-H. Wei*, Adv. Funct. Mater., 24, 7025 (2014).

[8] S. Yang, Y. Li, X. Wang, N. Huo, J.-B. Xia, S.-S. Li and Jingbo Li*, Nanoscale, 6, 2582 (2014).

[9] S. Yang, C. Wang, H. Sahin, H. Chen, Y. Li, S. S. Li, A. Suslu, F. M. Peeters, Q. Liu*, Jingbo Li*, S. Tongay*, Nano Letters, 15, 1660 (2015).

[10] B. Li, L. Huang, M. Zhong, N. Huo, Y. Li, S. Yang, C. Fan, J. Yang, W. P. Hu*, Z. M. Wei*, and Jingbo Li*, ACS Nano., 9，1257, 2015.

Currently there is a great challenge to create room temperature multiferroic materials with sizeable electric and magnetic dipole orderings. Most single-phase multiferroics to date have insignificant value either for one of such orderings or their mutual coupling. Here, we predict some new perovskite-derived materials which possess robust ferroelectric and magnetic properties, from the first principles calculations. These designed complex oxides might be tested experimentally and are expected to widen the bottleneck of the state-of-the-art spintronics technology.

 

Reference:

Predicting a ferrimagnetic phase of Zn₂FeOsO₆ with strong magnetoelectric coupling

P. S. Wang, W. Ren, L. Bellaiche, and H. Xiang, Phys. Rev. Lett., 114, 147204 (2015).

Creating multiferroics with large tunable electrical polarization from paraelectric rare-earth orthoferrites

Hong Jian Zhao, Yurong Yang, Wei Ren, Ai-Jie Mao, Xiang Ming Chen, and Laurent Bellaiche, J. Phys. Condens. Matter, 26, 472201 (2014).

Near room-temperature multiferroic materials with tunable ferromagnetic and electrical properties

Hong Jian Zhao, Wei Ren, Yurong Yang, Jorge Íñiguez, Xiang Ming Chen, and L. Bellaiche, Nature Communications, 5, 4021 (2014).

二维单原子层材料磁性的计算模拟研究

孙强

北京大学工学院材料系

北京大学应用物理研究中心

 

 

石墨烯因其独特的原子结构和新奇的物理化学特性激发了人们对单原子层材料极大的研究热情。探讨二维单原子层材料的磁性便是其中的一个重要研究课题。随着计算机硬件和软件技术的发展以及“材料基因工程”的实施，计算模拟已经成为新材料设计和合成的重要手段。本报告将重点讨论二维单原子层材料磁性的计算模拟研究，包括以下几个方面的内容：（1）石墨烯多孔化所引起的磁性；（2）BN单原子层和石墨烯杂化所引起的磁性；（3）表面修饰所引起的磁性；（4）金属有机单原子层材料的磁性； （5）单原子层MnX₂ (X=O, S, Se) 的磁性。对这些体系的讨论可以加深对单原子层材料中磁耦合机理及其调控的认识，从而为新型磁性材料的合成提供理论支撑。

 

The experimental observation of an abnormal Mott insulator to metal transition in 1T-TaS₂ was assumed to be generated by the interplay between correlation and disorder. Considering the stacking disorder in this compound, we take a theoretical analysis of the interplay of interactions and off-diagonal disorder in the one-dimensional extended Anderson-Hubbard model. We employ the real-space dynamical mean-field theory, which can treat correlation and disorder on an equal footing, to study the competition among the hopping disorder W’, on-site interaction U, and on-site disorder W. We find that when U=W=0, the density of states has a peak at E=0 with increasing hopping disorder. We use the generalized inverse participation ratio (GIPR) as a measure of the localization, and find that the system is localized at finite W'. We also find that when W=0 and U≠0, the density of states has a Mott gap, but also has a peak at the half of the band width at large W’. The lattice size scaling of the GIPR indicates that the localization effect of the off-diagonal disorder is much weaker than that of the on-site disorder. As a result, the Mott gap cannot be closed by the off-diagonal disorder, differing significantly from the diagonal disorder. With this method, we also discuss the dependences of the optical conductivity with U, W and W’.

    We carry out our first-principles calculations within density functional theory to study the 3d transition metal (TM) doped AlN nanosheets. The calculated results indicate that a stoichiometric AlN nanosheet is graphene-like structure and nonmagnetic. The TM impurities can induce magnetic moments, localized mainly on the 3d TM atoms and neighboring N atoms. Our calculated results of TM-doped nanosheet systems indicate a strong interaction between 3d orbit of TM atom and the 2p orbit of N atoms. In addition, the Mn- and Ni-doped AlN nanosheet with half-metal characters seems to be good candidates for spintronic applications. When substituting two Al atoms, the relative energies of the states between ferromagnetic and antiferromagnetic coupling are investigated sufficiently. The exchange coupling of Co- and Ni-doped AlN nanosheets exhibits a transformation with different distances of two TM atoms and that of Cr-, Mn-, and Fe-doped AlN nanosheets is not changed.

 

洪特耦合是导致非简并多轨道体系发生轨道选择Mott相变的重要因素之一, 本文通过调控洪特耦合来研究其不同组成部分对轨道选择Mott相变的作用. 我们利用基于Lanczos求解器的动力学平均场理论来研究双轨道Hubbard模型的金属-绝缘体相变, 并讨论了洪特耦合中的自旋翻转项和电子对跃迁项以及轨道宽度比值$w_2/w_1$如何来影响轨道选择Mott相变. 对比包含完整洪特耦合的J模型的相图, 我们发现Jz模型的轨道选择Mott相只存在于一个很狭窄的区域内, 说明自旋翻转项及电子对跳跃项是有利于轨道选择Mott相变发生的关键因素. 而当轨道宽度之比大于$w_2/w_1=0.7$时, Jz模型的轨道选择Mott相会完全消失. 因而, 简化后的Jz 模型只是在特定条件下才适合于研究轨道选择Mott相变.

Superior catalytic activity and high chemical stability of inexpensive electrocatalysts for the oxygen reduction reaction (ORR) are crucial to the large-scale practical application of fuel cells. The transition-metal/N modified graphene electrocatalysts are regarded as one of potential candidates but the further improvement of their activity and stability is limited by the unclear atomic mechanism during the ORR. Here, the ORR mechanism of Co-N codoped graphene electrocatalysts have been studied by using first-principles calculations and ab initio molecular dynamics simulations. Compared to pure N-doped and Co-doped graphene catalysts,  Co/N modified graphene catalysts exhibit higher stability and superior catalytic activity in the ORR process. The high stability of Co-N₄ complexes in graphene originates from that the Co-graphene interactions are modified by the hybridization between the Co-3d states and N-2p states. Moreover, this catalyst with a synergistic coupling effect between dopants leads to a smaller overpotential of ORR towards the four-electron transfer pathway relative to that of pure Co-doped and N-doped graphene electrocatalysts. Our results demonstrate that the Co-N codoped graphene is able to compete against the noble-metal electrocatalysts (such as Pt) and more effective graphene-based electrocatalysts may be realized by optimizing the coupling between transition-metal and non-metallic dopant elements.

    The detailed atomic structure of quasicrystals has been an open question for decades. Here, we present a quasilattice-conserved optimization method (quasiOPT), with particular quasiperiodic boundary conditions. As the atomic coordinates described by basic cells and quasilattices, we are able to maintain the self-similarity characteristics of qusicrystals with the atomic structure of the boundary region updated timely following the relaxing region. Exemplified with the study of decagonal Al-Co-Ni (d-Al-Co-Ni), we propose a more stable atomic structure model based on Penrose quasilattice and our quasiOPT simulations. In particular, “rectangle-triangle” rules are suggested for the local atomic structures of d-Al-Co-Ni quasicrystals.

摘要：近年来，有关纳米体系热输运性质及其调控的报道越来越多。在非常低的温度下，从理论和实验上相继发发现了由声子、电子、和光子导致的著名的量子化热导现象【1】,而且量子化热导单元不依赖于载流子类型。我们建立了低温下能处理各种维度的弹性声子热输运方程，从声子模式的角度系统地探索了一维、二维和三维纳米结构中热导量子化的本质【2】。在室温和高温下，声子扩散输运起主要作用，一些实验和理论工作研究了热量子器件的机理及其应用【3】。我们在热整流机理方面做了一系列探索【4，5】，发现量子器件热整流行为不同于电整流，电整流比可以达到非常高【6】，而很难找到热整流比非常高的热整流结构，研究人员对如何提高热整流比很感兴趣，我们在研究中发现了一种重要的热整流机理—声子驻波，当体系中驻波存在时，热整流比会大大提高，而且这个结论具有一定普适性【5】；我们设计了一种基于核壳纳米线的热缆，能够调控热的传输【7】；我们也探索了纳米尺度热电性能的声子调控机制【8】。最近，我们发展了一个计算公式，称之为混合的弹性-扩散声子热输运公式。根据这个公式，我们能够计算出一个临界温度，可区分声子的弹性和扩散热输运。 当温度在临界温度以下，主要是声子弹性输运对热导起作用，而当温度高于临界温度时，扩散输运开始对热导做出贡献。运用该公式计算了二维石墨烯纳米带和一维GaAs纳米线的热导，发现根据我们的公式计算的热导低于量子弹性方法计算的热导、高于经典扩散方法计算的热导，但定性符合实验结果，该公式适于所有晶体材料构筑的纳米尺度体系【9】。

参考文献:

[1] L. G. C. Rego and G. Kirczenow, Phys. Rev. Lett. 81, 232 (1998)；K. Schwab, et al., Nature 404, 974 (2000)； O. Chiatti, et al., Phys. Rev. Lett. 97, 056601 (2006)；M. Meschke, et al., Nature 444, 187 (2006).

[2] Ke-Qiu Chen*, Wen-Xia Li, W. H. Duan, Z. Shuai, and B. L. Gu, Phys. Rev. B 72, 045422 (2005)；Xiao-Fang Peng, Ke-Qiu Chen*,et al., Appl. Phys. Lett. 90, 193502 (2007).；Xiao-Fang Peng, Ke-Qiu  Chen*, Qing Wan, B. S.Zou, and Wenhui Duan, Phys. Rev. B 81, 195317 (2010)；Xiao-Fang Peng, Xin-Jun Wang, Zhi-Qiang Gong, and Ke-Qiu Chen*, Appl. Phys. Lett. 99, 233105 (2011)；Xiao-Fang Peng, Xin-Jun Wang, Li-Qun Chen, and Ke-Qiu Chen*, Eur. Phys. Lett. 98, 56001 (2012); Xiao-Fang Peng, Ke-Qiu Chen*, Carbon 77, 360 (2014).

[3] Baowen Li, et al., Phys. Rev. Lett. 93, 184301 (2004)；Jonghoon Lee et al. Nano Lett. 12, 3491 (2012)；Ruan et al. Nano Lett. 14, 592 (2014).

[4] Zhong-Xiang Xie, Ke-Min Li, Li-Ming Tang, Chang-Ning Pan, and Ke-Qiu Chen*, Appl. Phys. Lett. 100, 183110 (2012).

[5] Yue-Yang Liu, Wu-Xing Zhou, Li-Ming Tang, and Ke-Qiu Chen*, Appl. Phys. Lett. 105, 203111 (2014); Yue-Yang Liu, Wu-Xing Zhou, and Ke-Qiu Chen*, (Submitted to Appl. Phys. Lett. 2015).

[6] Dacheng Wei, Lanfei Xie, Kian Keat Lee, Zhibin Hu, Shihua Tan, Wei Chen, Chorng Haur Sow, Ke-Qiu Chen, Yunqi Liu, and Andrew Thye Shen Wee, Nature Commun. 4, 1374 (2013).

[7] Yue-Yang Liu, Wu-Xing Zhou, Li-Ming Tang, and Ke-Qiu Chen*, Appl. Phys. Lett. 103, 263118 (2013).

[8] Wu-Xing Zhou and Ke-Qiu Chen*, Scientific Reports, 4, 7150 (2014); Carbon 85, 24 ( 2015 ) .

[9] Xiao-Fang Peng, Ke-Qiu Chen*, et al. (Submitted to Phys. Rev. B).

 

       材料的宏观性质往往与其微观结构密切相关。因此确定材料的微观结构是开展功能材料设计的基础。但是，由于材料结构势能面的复杂性导致在只给定材料化学组分的条件下，从理论上确定材料的结构一直是一个极具挑战的难题。我们研究组基于粒子群多目标群智优化算法以及多种结构处理方法，发展了CALYPSO结构预测方法[1-2]，在此基础上开发了拥有自主知识产权的同名结构预测软件包（www.calypso.cn）。我们的方法和软件只需给定化学组分和外界条件就可以预测三维晶体[1]，零维的团簇或孤立分子[3]，表面重构[4], 二维层状材料[5-6]以及二维材料的吸附等体系的微观结构，并可以根据需要进行功能材料（超硬材料等）的结构设计[7]。

        最近我们将CALYPSO结构预测方法应用于金刚石(100)表面，意外发现了一类新奇的自组装碳纳米管阵列的表面重构，其结构类似于有序排列的碳纳米管通过强共价键镶嵌在金刚石的(100)表面上[4]。常规条件下，自组装碳纳米管阵列的表面结构与传统的二聚体表面结构的能量极其相近，但在高温或者压应力的外界条件下，碳纳米管阵列表面的能量更加稳定。碳纳米管阵列结构兼具金刚石高导热性和碳纳米管高载流子迁移率的特点，同时具有高的热稳定性，可自组装。本研究表明表面不仅可以用作衬底来集成器件，而且其重构体本身可以作为功能器件的一个重要部分具有功能的属性。研究有望实现人们长期期盼的将金刚石与碳纳米管的有机结合，在金刚石基半导体器件研究上具有潜在的价值。

参考文献:

[1] Y. Wang, J. Lv, L.Zhu, and Y. Ma, Phys. Rev. B, 2010, 82, 094116.

[2] Y. Wang, J. Lv, L.Zhu, and Y. Ma, Comput. Phys. Commun. 183, 2063 (2012).

[3] J. Lv, Y. Wang, L.Zhu, and Y. Ma, J. Chem. Phys. 137, 084104 (2012).

[4] S. Lu, Y. Wang, H. Liu, M. Miao, and Y. Ma, Nature Commun. 5, 3666 (2014)

[5] Y. Wang, et al., J. Chem. Phys. 137, 224108 (2012).

[6]X. Luo, et al., J. Am. Chem. Soc. 133, 16285 (2011).

[7]X. Zhang, et al., J. Chem. Phys. 138, 114101 (2013).

Electrides are a special kind of ionic solids with cavity-trapped electrons serving as the anions. Since their first discovery in 1983, several electride materials have been identified or synthesized. These materials possess special physical and chemical properties which are sensitive to the topology and geometry of the cavities which confine the anionic electrons. Recently, a new type of electride materials has been discovered, for which the confining cavities have 2D planar geometry, hence they are termed as 2D electrides. In this talk, I will introduce our recent work on a 2D electride, Ca₂N. We find that due its special property, this material, in its bulk form or few-layer structures, could have great potential for optical, plasmonic, and energy storage applications. 

Graphene is a 2D hexagonal lattice made of sp² hybridized carbon. Fundamental understanding of graphene has recently spurred a surge of searching for 2D topological quantum phases in solid-state materials. Here we demonstrate the epitaxial growth of artificial graphene, in which the carbon atoms are replaced by other elements, on conventional semiconductor surface to realize large-gap topological quantum phases. We show that Si(111) surface functionalized with 1/3 monolayer of halogen atoms [Si(111)-sqrt(3)×sqrt(3)X(X=Cl, Br, I)] exhibiting a trigonal superstructure, provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Remarkably, the Bi overlayer show the feature of a (p_x, p_y) analogue of graphene that exhibits quantum spin Hall state with an energy gap as large as ~ 0.8 eV.^1,2 Growth of transition metals lead to the discovery of a new 2D material, sd² graphene,³ characterized with bond-center electronic hopping, which surprisingly transforms the atomic hexagonal lattice into a hidden Kagome lattice and exhibits a wide range of topological quantum phases. For example, quantum anomalous Hall states can be realized in W@ Si(111)-sqrt(3)×sqrt(3)-Cl, with an energy gap of ~ 0.1 eV. These findings may pave the way for future exploration of graphene-based topological quantum phases, by exploiting epitaxial growth and current available semiconductor technology.^4,5

1. M. Zhou, W. Ming, Z. Liu, Z. Wang, P. Li, and F. Liu, Proc. Natl. Acad. Sci. U. S. A 111, 14378 (2014).
2. M. Zhou, W. Ming, Z. Liu, Z. Wang, Y. Yao, and F. Liu, Sci. Rep. 4, 7102 (2014).
3. M. Zhou, Z. Liu, W. Ming, , Z. Wang, and F. Liu, Phys. Rev. Lett. 113, 236802 (2014).
4. M. Zhou, Z. Liu, Z. Wang, Z. Bai, Y. Feng, M. G. Lagally, and F. Liu, Phys. Rev. Lett. 111, 246801 (2013).
5. M. Zhou, et al. Int. J. Mod. Phys. B (invited review), In preparation (2015).

在纳米体系的合成过程中，温度对体系的形状、尺寸有着重要的影响。在低温条件下，熵对体系的自由能的贡献远小于总能，而不同体系的总能与结构最相关；在高温条件下，熵对体系的自由能的贡献越来越明显，这是实验中不同温度下观察到不同稳定结构的主要原因。

以碳族纳米团簇为例[1]，我们采用随机搜索、结合第一原理方法，系统研究了不同表面修饰对碳族纳米材料结构和性质的调制，关注不同表面原子、基团修饰对体系形状、表面、内部原子分布等的影响等。根据体系的成键特性区分不等价结构，我们采用凸点分析的方法获得了和实验一致的幻数结构[2]；对于硅锗纳米结构，高温条件下我们重点关注混合熵的贡献[3]。另外，我们将结合对硼平面团簇的研究[4]探讨如何实现纳米结构的高通量计算。

参考文献:
[1] Xiaobao Yang*, Yu-Jun Zhao, Hu Xu, Boris I. Yakobson, Phys. Rev. B 83, 205314(2011).
[2]Hongdo Lu, Yu-Jun Zhao, Xiao-Bao Yang*, and Xu Hu*, Phys. Rev. B 86, 085440(2012).
[3]Xiao-Bao Yang*, Yu-Jun Zhao, and Hu Xu*, J. Chem. Phys. 139, 154713 (2013).
[4]Shao-Gang Xu, Yu-Jun Zhao, Ji-Hai Liao, and Xiao-Bao Yang*, J. Chem. Phys. 142, 214307(2015).

Electronic Properties of In-plane Heterostructures of Phosphorene and Graphene

 

Xiaoqing Tian,¹ Lin Liu,¹ Yadong Wei, ¹ Yu Du,¹ Juan Gu, ¹ Boris I. Yakobson,^2,* and Jian-bin Xu ³

 

¹College of Physics and Technology, Shenzhen University, Shenzhen 518060, Guangdong, P. R. China.

²Department of Mechanical Engineering and Materials Science, Department of Chemistry, and the Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas 77005, United States.

³Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong SAR, P. R. China.

 

*E-mail: biy@rice.edu

 

Phosphorene and graphene have a tiny lattice misfit along the armchair direction, which could result in an atomically sharp in-plane interface. The electronic properties of in-plane heterostructures of phosphorene/graphene are investigated by the first-principles method. Here we demonstrate that the electronic properties of this type of heterostructure can be highly tunable by the quantum size effects and the E_ext. The transition of semiconducting to Dirac materials by the E_ext reveals a new routine to explore massless carriers. Given these results, it is expected that in-plane heterostructures of phosphorene/graphene will present abundant opportunities for applications in optoelectronic and electronic devices.

目前，固态基底上薄膜的去湿润行为已经得到了广泛的理论和实验研究，得到了许多有意义的研究结果。而不同的薄膜材料，如水分子，高分子聚合物，金属等，由于不同的原子结合特征和表面特性，其去湿润行为表现出了明显的差异。金属薄膜因其具有高表面能，低粘性系数，长程相互作用力等特点使其在固态基底表面动力学演化的去湿润和脱离行为明显不同于水分子和高分子有机聚合物等软物质材料。一般来讲，纳米薄膜在固态基底表面的动力学演化过程不仅与环境的温度和压力等因素有关，而且与纳米薄膜的厚度，尺寸，甚至形状等参数也有关。近年来，探讨纳米薄膜的去湿润行为与这些因素的依赖关系是纳米材料研究领域的热点问题。
我们采用分子动力学的方法研究了金属Pt纳米薄膜在固态石墨基底表面的动力学演化过程。通过构建石墨基底上不同层数的金属Pt圆盘纳米薄膜，探讨金属纳米薄膜在固态基底表面的去湿以及脱附的动力学演化过程对薄膜初始厚度、温度以及基底层数等的依赖关系。我们的研究结果表明，在一定的温度条件下，不同初始厚度的Pt纳米薄膜存在不同的去湿现象。由于Pt原子层和石墨基底之间有较强的相互作用，层数较小的纳米薄膜在去湿过程中会形成纳米空洞，并且完全脱离基底表面需要较高的温度；而层数较大的纳米薄膜的去湿过程则从薄膜边缘开始收缩，直到最终形成纳米液滴，以一定的速度脱离基底表面。进一步的研究表明，随着金属纳米薄膜初始层数的增加，液滴脱离基底表面的速度先增大后减小，这一结果我们通过去湿过程中金属的表面能和金属与基底之间的相互作用能之间的竞争关系进行了理论分析。此外，我们还通过比较Au和Pt的圆盘纳米薄膜的去湿和脱附过程，探究金属薄膜与基底间的粘滞耗散对液膜去湿的影响。
这些研究结果不仅对于理解其他金属纳米薄膜在固态基底表面的动力学演化过程具有理论意义，而且对于金属镀膜，表面清洁，器件表面去湿等工业生产过程具有一定的现实意义。

    探索高压下碱土金属碳化物的结构和电子特性的研究是近年来凝聚态物理领域研究热点，一方面可以理解插层化合物在高压下的超导特性，另一方面可以探索新型碱土金属碳化物的可能。采用遗传算法结合第一性原理总能计算和高压实验合成，我们探讨了钙-体系在高压下的结构相变及其电子特征。主要结论包括：钙碳化物在高压下可发生碳聚合，部分碳化物呈现出超导特性^[1-4]；从CaC₆高压新相，通过去除钙原子可以设计三维的纳米孔结构，部分设计的纯碳结构呈现出压缩膨胀现象^[5]；对于CaC₆，在高压下超导消失，进一步压缩下，超导可以再现^[2]；发现一些新型的碳化物(Ca₅C_2, Ca₂C, Ca₃C₂, Ca₂C₃, CaC)，部分结构已经被实验所证实(Ca₂C 和Ca₂C₃)^[6]。研究结果表明，利用极端条件，借助碱土金属，不但可以合成新型碳结构，而且可以合成具有特别电子特性的新型碳化物。

参考文献

1. Yan-Ling Li, Wei Luo, Zhi Zeng, Hai-Qing Lin, Ho-kwang Mao, and Rajeev Ahuja. Pressure-induced superconductivity in CaC₂. Proceedings of the National Academy of Sciences 110(23):9289-9294 (2013).

2. Yan-Ling Li, Wei Luo, Xiao-Jia Chen, Zhi Zeng, Hai-Qing Lin, and Rajeev Ahuja. Formation of Nanofoam carbon and re-emergence of Superconductivity in compressed CaC₆. Sci. Rep. 3:3331 (2013).

3. D. Benson, Yan-Ling Li, W. Luo et al. Lithium and Calcium Carbides with Polymeric Carbon Structures. Inorganic Chemistry 52(11):6402-6406 (2013).

4. Yan-Ling Li, Rajeev Ahuja, and Hai-Qing Lin. Structural phase transition and metallization in compressed SrC₂. Chinese Science Bulletin (Letter) 59(36),5269-5271 (2014). （约稿）

5. X. Jiang, J. Zhao, Yan-Ling Li, and R. Ahuja. Tunable Assembly of sp³ Cross-Linked 3D Graphene Monoliths: A First-Principles Prediction. Advanced Functional Materials 23(47):5846-5853(2013).

6. Yan-Ling Li, Sheng-Nan Wang, Artem R. Oganov, Huiyang Gou, Jesse S. Smith, and Timothy A. Strobel. Investigation of Exotic stable calcium carbides using theory and experiment. Nat. Commun. 6, 6974 (2015).

    

   致谢: 本项目得到江苏省”青蓝工程”中青年学术带头人基金和国家自然科学基金资助(11047013, 11347007).

In this talk, some progresses on the study of silicene and its ribbon by using first-principles pseudopotential calculations within density-functional theory and containing SOC will be reported. We firstly study the stabilities, micro-geometries and electronic properties of alkali metals atoms adsorption on silicene. In addition, we performed the comparison of our results with that of pure silicene and hydrogen saturated silicene. We found that all the formation energies of saturated adsorption of alkali metal atoms on silicene SiX(X=Li, Na, K and Rb) are negative, indicating that the relative stabilities of these new compounds when compared with silicene. Bader charge analysis shows that charge is transferred from Si atoms to H atoms in SiH compound, but in SiX the direction of charge chansfer is opposite, i.e., the charge is transferred from alkali metals atoms to Si atoms. From the viewpoint of chemical bonding, we found that valence bond is formed between Si atoms and H atoms, but the bonds between Si atoms and alkali metal atoms are mainly ionic. From the band structure calculations, we found that the SiH open an indirect band gap, the new type compound SiLi is a semiconductor with a direct band gap of 0.34eV. However, all the other compounds SiX(X=Na, K and Rb) exhibit metallic properties. Furthermore, the recent calculated results of adatoms adsorption on the edges of silicene ribbon will be introduced.

This work is supported by National Natural Science Foundation of China (No. 11374251).

 

    Bi₂Se₃和Bi₂Te₃是第三代强拓扑绝缘体，其优异性能都与表面的Dirac锥有关。但是实验发现Bi₂Se₃和Bi₂Te₃薄膜样品的Dirac点在Fermi能级以下且间隔较大。这往往归结于样品中存在的施主缺陷(例如Se和Te空位和反位缺陷)造成的Fermi能级上升。这对拓扑绝缘体的应用是非常不利的，大量的块体载流子和Dirac费米子混合在一起，使后者的优越性质被掩盖而探测不到。基于这样的观点，目前往往采取掺杂受主杂质(acceptor impurities)的方法使Fermi能级下移到Dirac点。但是这会引入很多杂质态。

    减少Fermi能级上移的另一个途径是减少缺陷。但是实验发现，生长在graphene和Si(111)衬底上的高质量(缺陷很少)Bi₂Se₃和Bi₂Te₃薄膜的Dirac点仍然明显在Fermi能级以下，且随着薄膜生长的厚度增加费米能级越接近Dirac点。这个费解的问题一直没有得到解释。

    我们的计算研究发现，Bi₂Se₃和Bi₂Te₃在没有衬底的情况下Fermi能级是十分接近Dirac点的，与实验中有衬底时Fermi能级明显高于Dirac点的结果比较，说明衬底这个以前被忽视的因素可以对拓扑绝缘体Fermi能级的位置可以产生重要影响。我们用第一性原理方法研究了Bi₂Se₃/graphene和Bi₂Te₃/Si(111)体系的性质，发现计算中考虑衬底后，Fermi能级移到了Dirac点的位上面，与实验符合得很好。通过计算功函数和电荷密度，发现graphene和Si(111)的功函数比Bi₂Se₃和Bi₂Te₃薄膜的小很多，电荷从衬底转移到Bi₂Se₃和Bi₂Te₃薄膜上，使Fermi能级向上移动。由于电荷的转移量几乎不随Bi₂Se₃和Bi₂Te₃薄膜的厚度增加而变化，而当厚度增加时，在Fermi能级附近的态密度增加，因此由电荷转移而引起的Fermi能级移动随拓扑绝缘体薄膜厚度增加而减小，与实验符合得很好。

    基于上述对衬底作用的计算，我们提出提高衬底功函数使之高于Bi₂Se₃和Bi₂Te₃薄膜的功函数，使电荷转移反转，从而降低费米能级使之接近Dirac点。计算发现，在graphene和Si(111)面上吸附F原子可以使衬底的功函数显著高于Bi₂Se₃和Bi₂Te₃薄膜的功函数，实现了电荷转移的反转，使Fermi能级有效降低。通过调整衬底功函数而不是通过掺杂来实现Fermi能级的调控的方案，为拓扑绝缘体的应用提供了一个新的途径。

Based on first-principles DFT calculations, we have proposed and studied two novel topological insulators that are derived from layered materials, e.g. the single-layer structure and the van der Waals heterostructure. The first one is the monolayer of bismuth monobromide (Bi4Br4). Our calculations show that the monolayer Bi4Br4 is 2D Topological insulator (or QSH insulator) with a sizable band gap of about 180 meV. The physics of band inversion was investigated by using the low-energy k*p effective Hamiltonian and atomic orbitals evolution analysis. Further studies on the multilayer structure reveal that the interlayer coupling has very small effect on the low-energy band structure, hence the monolayer steps on the surface of Bi4Br4 single crystal are adequate for experimental measurement of topological edge states. The other one is the van der Waals heterostructure of BiTeI/Bi2Te3. We find that the intercalation of Bi2Te3 into BiTeI result in an interesting inversion asymmetric TI which also has giant Rashba spin splitting (RSS). The surface electronic structure and spin texture in BiTeI/Bi2Te3 present unique features due to the coexistence of TI and RSS. 

References:
[1] Zhou, J.-J., Feng, W., Zhang, Y., Yang, S. A. & Yao, Y. Engineering Topological Surface States and Giant Rashba Spin Splitting in BiTeI/Bi2Te3 Heterostructures. Sci. Rep. 4, 3841 (2014).
[2] Zhou, J.-J., Feng, W., Liu, C.-C., Guan, S. & Yao, Y. Large-Gap Quantum Spin Hall Insulator in Single Layer Bismuth Monobromide Bi4Br4. Nano Lett. 14, 4767–4771 (2014).
[3] Zhou, J.-J., Feng, W., Liu, G.-B. & Yao, Y. Topological edge states in single- and multi-layer Bi4Br4. New J. Phys. 17, 015004 (2015).

Activation of O₂ molecules by transition metal (TM) catalysts is vital to CO oxidation in industrial applications. Recent experimental progresses have advanced the field towards single-atom catalysis on different substrates, but the underlying microscopic mechanisms operative in such new class of catalytic systems remain elusive. Here, we use first-principles density functional theory calculations to investigate O₂ activation on a monomer or dimer of two representative TMs (Pd and Ni) deposited on a defective TiO₂(110) (TM_n@TiO₂(110); n=1,2) as prototypical examples. We find that the activities of such atom-sized TM catalysts for O₂ activation can be well described within a generalized d-band model that emphasizes the gap between the frontier orbitals of TM and the O₂ molecule, rather than by the position of the d-band center of a given TM relative to the Fermi energy as depicted in the classic d-band theory. Specifically, a Pd₂@TiO₂(110) catalyst with a relatively small HOMO(TM)-LUMO(O₂) gap, compared to Pd@TiO₂(110), enables the HOMO of Pd₂ to serve both as a charge reservoir to donate electron charge to the O₂ molecule and as a spin reservoir to accommodate electron spin from O₂, therefore enhancing the O₂ activation. The present findings successfully explain why distinct catalytic activities between Pd@TiO₂(110) and Pd₂@TiO₂(110) were observed in atom-sized catalysis [Kaden, et al., Science 326, 826 (2009)]. More significantly, the generalized d-band model proposed here is also anticipated to play an instrumental role in future design of exotic atom-sized catalysts.

金属纳米颗粒在环境能源领域有着重要的作用。近年来，研究发现合金纳米颗粒在一系列重要反应中表现出了优异的特性。为了优化材料的性能，需要理解纳米材料活性的来源，从而设计材料的组份，控制材料的表面结构。密度泛函理论（DFT）结合微观动力学模拟（MKM）以及动力学蒙特卡洛（KMC）模拟为这样的研究提供了可能，因为功能材料的设计需要从微观基元反应出发，过渡到纳米甚至微米量级的颗粒表面，从而得到宏观可测的量。这里，我通过一氧化氮和氨气在合金表面的分解，介绍多尺度模拟在纳米催化材料设计中的应用。DFT结合MKM, KMC的多尺度模拟方法，可以应用到其他的复杂反应和过程中，从而为材料的设计提供指导。

Stanene is a two-dimensional (2D) atomically thin sheet of tin in a buckled honeycomb lattice. In contrast to other 2D group-IV materials (including graphene, silicene and germanene), stanene is unique by its exotic properties. The material can support many novel features, including large-gap quantum spin Hall (QSH) states, quantum anomalous Hall (QAH) states, enhanced thermoelectric performance, or possibly even topological superconductivity. Recent progresses on the research of stanene will be reviewed. In particular, as an important progress, a recent experiment has successfully fabricated monolayer stanene, which, for the first time, confirmed the existence of this theoretically predicted structure. All of these progresses make the research of stanene as a fast-growing field.

Titanium dioxide materials have been studied intensively and extensively due to photocatalytic applications. A long-standing open question is the energy band alignment of rutile and anatase TiO₂ phases, which can affect the photocatalytic process in the composite system. There are basically two contradictory viewpoints about the alignment of these two TiO₂ phases supported by respective experiments: 1) straddling type and 2) staggered type. In this work, our DFT plus U calculations find that the clean rutile (110) and anatase (101) surfaces have the straddling type band alignment, whereas the surfaces with defects can turn the band alignment into the staggered type. The electric dipoles induced by defects are responsible for the reversal of band alignment. Thus the defects introduced during preparations and post-treatment processes of materials are probably the answer to above open question regarding the band alignment, which can be considered in real practice to tune the photocatalytic activity of materials.

Semiconductor Materials Genome Initiative: Inverse Design Silicon Nanomaterials

Jun-Wei Luo*,Shu-Shen Li

State Key Laboratory of Superlattice and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

*jwluo@semi.ac.cn

 

Keywords: materials genome, inverse design, Si-based light emission, valley splitting, Si spintronics,

Abstract: The Materials Genome Initiative (MGI) is lunched to help businesses discover, develop, and deploy new materials twice as fast. Semiconductors playing a key role in a wide assortment of technologies and industries ranging from economy, human well being, and national security are essential part of the materials genome initiative. Here we present demos of semiconductor materials genome to accelerate the development of Si-based light emission and spintronics. Receipts for optics-friendly silicon. Si is the darling of the microelectronics industry, but it has an Achilles’ heel: Si can’t absorb or emit light without the help of phonons. This so-called indirect band gap makes it an inefficient option for light-emitting diodes and solar cells. On the other hand, there is an urgent requirement for an optical emitter that is compatible with standard, silicon-based ultra-large-scale integration technology. The discovered Si/Ge magic sequence superlattices [1] or nanowires [2] or core/multishell nanowries [1] exhibit orders more efficient at absorbing (emission) light than their existing counterpart records and approach more than 10% brightness of real direct gap materials. In principle, such magic sequence superlattice could be prepared with molecular beam epitaxy, and the nanowire can be prepared by selective area metalorganic vapor phase epitaxy. Enhanced valley splitting towards a spin qubit in Si. Electronic spins in Si are raising contenders for qubits -- the logical unit of quantum computation-- owing to its outstanding spin coherence properties and compatibility to standard electronics. A remarkable limitation for spin quantum computing in Si hosts is the orbital degeneracy of this material's conduction band, preventing the spin-1/2 states from being an isolated two-level system. Very recently, we numerically inverse designed Si quantum well to isolating a single electron valley state in Si by a magic-sequence of Ge/Si barrier layers [3].

 

 

References:

[1] M. d’Avezac , J.W. Luo, and Zunger, Phys. Rev. Lett. 108, 027401 (2012);  L. Zhang, M. d’Avezac, J.W. Luo, A. Zunger, Nano Lett. 12, 984 (2012)

[2] X.W. Jiang, J.W. Luo, and S.S. Li, manuscript in preparation.

[3] L. Zhang, J.W. Luo, A.L. Saraiva, B. Koiller, A. Zunger, Nature Communications 4, 2396 (2013).

 

Traditionally the origin of the poor p-type conductivity in some transition metal oxides (TMOs) was attributed to the limited hole concentration: the charge-compensating donor defects, such as oxygen vacancies and cation interstitials, can form spontaneously as the Fermi energy shifts down to near the valence band maximum. Besides the thermodynamic limit to the hole concentration, the limit to the hole mobility can be another possible reason, e.g., the hole carrier can form self-trapped polarons with very low carrier mobility. Although isolated hole polarons had been found in some TMOs, the polaron-polaron interaction is not well-studied. Here we show that in TMOs such as TiO₂, V₂O₅ and MoO₃, the hole polarons prefer to bind with each other to form double-hole bipolarons, which are more stable than free hole carriers or separated polarons. This pushes the hole states upward into the conduction band and traps the holes. The rise of the Fermi energy suppresses the spontaneous formation of the charge-compensating donor defects, so the conventional mechanism becomes ineffective. Since it can happen in impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs.

I will present the Dissipaton Equation of Motion (DEOM) formalism [1], together with its numerical advancement [2], for open quantum systems. It provides a unified treatment, in terms of fermonic or bosonic or excitonic (hard-core bosonic) dissipatons, for three universal classes of bath. While recovering the hierarchical equations of motion formalism [3], DEOM identities the previous auxiliary quantities to the quasi-particles dynamics of hybridizing bath. It offers a nonperturbative and universal means, exact in principle, for the study of steady-state (equilibrium or nonequilibrium) and real-time dynamical properties of both systems and hybridizing bath. Some benchmark evaluations will be presented. These include the linear and nonlinear Fano polarization spectroscopy [4], the quantum transport current noise spectrum and other structural and dynamical properties of quantum impurity systems in the Kondo regime [5], and the use of DEOM as an impurity solver in the dynamical mean-field theory [6].

[1] Y. J. Yan, J. Chem. Phys. 140, 054105 (2014).

[2] D. Hou, S. K. Wang, R. L. Wang, L. Z. Ye, R. X. Xu, X. Zheng, and Y. J. Yan, J. Chem. Phys. 142, 104112 (2015).

[3] J. S. Jin, X. Zheng, and Y. J. Yan, J. Chem. Phys. 128, 234703 (2008); Y. Tanimura, J. Phys. Soc. Jpn. 75, 082001 (2006); R. X. Xu, P. Cui, X. Q. Li, Y. Mo, and Y. J. Yan, J. Chem. Phys. 122, 041103 (2005).

[4] H. D. Zhang, R. X. Xu, X. Zheng, and Y. J. Yan, J. Chem. Phys. 142, 024112 (2015).

[5] J. S. Jin, S. K. Wang, X. Zheng, and Y. J. Yan, J. Chem. Phys. (2015; accepted);  Z. H. Li et al., Phys. Rev. Lett. 109, 266403 (2012); S. K. Wang, X. Zheng, J. S. Jin, and Y. J. Yan, Phys. Rev. B 88, 035129 (2013); X. Zheng, Y. J. Yan, and M. Di Ventra, Phys. Rev. Lett. 111, 086601 (2013); L. Z. Ye, D. Hou, R. L. Wang, D. W. Cao, X. Zheng, and Y. J. Yan, Phys. Rev. B 90, 165116 (2014).

[6] D. Hou, R. L. Wang, X. Zheng, N. H. Tong, J. H. Wei, and Y. J. Yan, Phys. Rev. B 90, 045141 (2014). 

 

 

Silicene nanostructures have both rich electronic and magnetic properties, and thus have potential applications for silicon-based electronic and spintronic nano-devices.  In this talk, I will show our results of the effects of thermal and electric fields on the electronic properties of silicene nanostructures based on first-principles calculations and statistical analysis of canonical ensembles. The effects of electric field on the energy gap and the edge bands near the Fermi level of silicene nanoribbons are discussed. We find that the electric field can induce the gapless spin semiconductor of silicene nanoribbons. The electric field induced features can be achieved by a suitable uniaxial compressive strain. This can be understood from the effect of the Wilson transition. I will also discuss the magnetic properties of the silicene doped with Fe and Cr metal atoms under external electric field. We find that the doped systems show a variety of interesting magnetoelectric behaviors. The magnetic moment of Fe doped silicene show a sharp jump at a threshold electric field, which indicates a good switching effect. The magnetic moment of the doped systems has a nearly linear region with the electric field. We find that the changes of magnetic moment strongly depend on the direction of the electric field. In particular, one configuration of Fe doped silicene shows an interesting magetoelectric response which can be considered as a magetoelectric diode.  At last, I will show the effects of thermal fields on the electronic properties of silicone and come to the following conclusions: i) The band structure features near the Fermi energy in the pristine silicene are protected in thermal field, which means that the high transferability of the Dirac electrons is retained. ii) The modulation of the electronic properties of silicene in a thermal field needs much smaller electric field than the silicene without thermal field. The higher temperature corresponds to a larger gap under the same electric field.

It is known that the kinetics is far more important than thermodynamics in the controlling of crystal growth in three-dimensional materials. However, our knowledge about the kinetic growth of low-dimensional materials is still very limited. Here by using two examples, two-dimensional (2D) graphene and one-dimensional (1D) carbon nanotube, to demonstrate the importance of kinetics in low dimensional material’s growth process.

1. The growth of graphene follows the kinetic Wulff construction (KWC) while the interaction between graphene and substrate plays another crucial role and determines the direction dependent growth rate. Here I will briefly introduce the domain shape determination in graphene CVD growth and etching of graphene domain;
2. Different from 2D and 3D materials, 1D material has only one direction of growth and thus the KWC can’t be applied. Here demonstrate that the growth of CNTs can be hundreds to thousands times faster than that of graphene because of the failure of the KWC. Besides, the zigzag type of SWCNTs grows slowest among all SWCNTs.

 

电子能带的计算一般是考虑晶体的平移原胞，利用平移对称性，构造布洛赫波，解决本征值问题。但是，许多准一维纳米材料的基本对称性并非平移对称，而是螺旋对称，比如纳米管，纳米线等。对于这类体系（在纳米领域很常见）而言，主流的能带方法由于需要考虑较大原子数的原胞而其效率变得很低。我们在过去的工作中发展了基于密度泛函紧束缚的广义布洛赫方法。这个新方法恰恰是利用了一维体系的螺旋对称，从而达到简化能带计算的目的。在这个报告中，我将介绍这个方法以及几个典型的应用实例。最近，我们的课题组又在自洽密度泛函紧束缚里面实现了广义布洛赫方法——这代表我们在这个方向上的新进展。  

As a neighbor for carbon, boron is also expected to have similar or even better electronic properties than carbon. Since boron sheets are a precursor for nanotubes and fullerenes, understanding their structure and stability is a prerequisite for understanding boron nanomaterials. Present results, based on the evolutionary structure prediction, provide new insights into the formation of two dimensional (2D) boron sheets with nonzero thickness. The non-flatness of 2D boron enhances its energetic stability. In particular, we found that a 2D-boron with Pmmn symmetry can exhibit massless Dirac fermions, which may be superior to graphene. We also predicted that some 2D boron sheets constructed by unusual cages (which are different from the well-known B₁₂ icosahedra) have novel electronic properties, such as the flat band near or at the Fermi level.

地幔矿物在超高温和超高压条件下的热输运性质是研究地球内部过程及演化的重要参数。但是由于实验条件的局限以及地幔矿物组成和结构的复杂性，目前对这些关键的材料性质的估计仍然存在很大的不确定性。更重要的是目前估计的热导率的最高值和最低值分别支持截然不同的地球演化理论。在今天的报告中， 我首先介绍我们近来在声子散射和弛豫时间的第一性计算方面的进展［1，2，3］。为了准确的预测复杂的地幔矿物的热导率，我们发展了高效率，大规模并行计算的数值方法， 计算了钙钛矿(MgSiO₃ perovskite, 20 atoms per unit-cell)和氧化物(MgO)在整个布里渊区中的所有声子弛豫时间，并且考虑了铁(Fe)原子对镁(Mg)原子的参杂效应, 以及氧化物和钙钛矿的复合材料平均。基于这些第一性计算结果，我们估测地球在核幔边界区域的平均总热导率 介于2.5-3.5 W/m/K。接下来， 我将介绍我们最新的collective 弛豫时间理论［4］，并在这个理论的框架内讨论Peierls 的kinetic 声子输运公式和Boltzmann的 声子输运公式 在不同温度下的差别。 最后， 我将讨论我们对高温条件下红外声子和红外光子耦合所产生的diffusive bulk 声子极化子的实验和理论研究数据， 以及相应的一个新的高温条件下热导机制 ［5］。

 

* The reported research work is financially supported by NSF grants awarded to Dong (EAR-0757847 and EAR-1346961).

 

[1] X. Tang and J. Dong, Phys. Earth Planet. Int 174, 33 (2009).

[2] X. Tang and J. Dong, Proc. Natl. Acad. Sci., U.S.A. 107, 4539 (2010).

[3] X. Tang et al. Geophy. Rev. Lett. 41, 2746 (2014).

[4] J. Dong et al, to be submitted to Phys. Rev. B.

[5] A.M. Hoffmeister, J. Dong, and J.M. Branlund, J. Appl. Phys. 115, 163517(2014).

在本报告中，我们将主要在量子反常霍尔效应可能应用以及最新的理论机制方面进行介绍，具体包括：（1）将基于石墨烯的量子反常霍尔效应体系置于超导体上，我们发现这样的复合结构可以用来设计高效的库伯对分离器，使得分离效率最高达到100%。而且该库伯对分离器对器件尺寸、超导体的相干长度以及外部杂质的影响都非常robust。（2）在目前广泛研究的量子反常霍尔效应体系中，大家普遍考虑的是SigmaZ型的塞曼场（即沿着垂直于面外的铁磁性）都是铁磁序沿着z轴的情况，而很多实际的磁型绝缘体本身都是具有面内磁性，因此我们提出了一种新的利用面内磁矩在石墨烯材料中来实现量子反常霍尔效应的物理机制。

Understanding the electronic dynamics on material surfaces or at interfaces of adsorbate-surface systems is crucially important to a variety of applications in physics and chemistry, such as photovoltaic conversion, heterogeneous catalysis, and scanning tunneling microscopy. However, theoretically it is very challenging to simulate the electronic dynamics at material surfaces/interfaces at atomic level. One major difficulty is that the conventional first-principles methods, particularly those based on the time-dependent density-functional theory (TDDFT), were developed mainly for isolated or periodic systems, while the surface of bulk material has an open boundary and is thus beyond the realm of conventional TDDFT methods [1, 2].

To overcome the above difficulty, we develop a practical TDDFT approach for systems with open boundaries [3]. The new practical approach for open systems thus greatly extends the applicability of TDDFT. The accuracy and practicality of our novel TDDFT approach is exemplified by simulations on two prototypical systems: (1) the dynamic relaxation of an excess electron on the two-dimensional surface of a graphene monolayer, and (2) the real-time electron transfer at the interface of a molecule-graphene complex. Both the transient and long-time asymptotic dynamics are examined. The numerical results clearly affirm the correctness and usefulness of our open-system TDDFT approach. The simulations also provide important insights into the characteristic features of temporal electron evolution and dissipation on surfaces of bulk materials.

References

[1] X. Zheng, F. Wang, C. Y. Yam, Y. Mo, and G. H. Chen, "Time-dependent density-functional theory for open systems", Phys. Rev. B, Vol. 75, 195127, 2007.

[2] X. Zheng, G. H. Chen, Y. Mo, S. Koo, H. Tian, C. Y. Yam, and Y. J. Yan, "Time-dependent density-functional theory for quantum transport", J. Chem. Phys., Vol. 133, 114101, 2010.

[3] R. Wang, D. Hou, and X. Zheng, "Time-dependent density-functional theory for real-time electronic dynamics on material surfaces", Phys. Rev. B, Vol. 88, 205126, 2013.

 

Shao-Gang Xu ¹, Yu-Jun Zhao^1,2, Ji-Hai Liao¹ and Xiao-Bao Yang^{1, 2*}

 

 

  Using the Density Functional Theory, we analyze the potential barrier, charge transfer and atomic orbital overlap at the metal- black phosphorus interface in an optimized structure to understand how efficiently carriers could be injected from metal contact to the black phosphorus. we investigate a monolayerblack phosphorus directly contact with Ag(111), Au(111), Al(111), Cu(111) or Zn(1000) five representative metal substrates having varying work functions but each with minimal lattice mismatch with the black phosphorus overlayer. We find that the contact nature are Ohmic versus Schottky. For different kinds of contact, Cu and Al show the better conductivity than the other metals. For the dependence of the barrier height exhibits a character of partial Fermi-level pinning. These findings may prove to be instrumental in future design of BP-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

Quantum anomalous Hall (QAH) effect, with potential applications in low-power-consumption electronics, is predicted in the heterostructures of graphene on an antiferromagnetic insulator (RbMnCl3) or a ferromagnetic insulator, based on density-functional theory and Wannier function methods. Due to the interactions from the substrate, a much large exchange field (about 280 meV) and an enhanced Rashba spin-orbit coupling can be induced in graphene, leading to a topologically nontrivial QAH gap opened in the system. Interestingly, in certain cases, the obtained QAH effect can be independent of the stacking patterns between the graphene and the substrate. And the nontrivial band gap is opened exactly around the Fermi level. These behaviors make the experimental observation of the QAH effect in the heterostructure straightforward. The avenues of enhancing the nontrivial gap are also proposed, from which nearly one order large gap is achieved. Our work demonstrates that the graphene-based heterostructures are appropriate candidates to be employed to experimentally observe the QAH effect and explore the promising applications.

 

 

在体相和受限条件下已经发现了很多种水结构，这反映了水分子形成多种多样氢键网络的超常能力。理想的氢键网络应该满足三个条件：1、每个水分子周围有4个氢键；2、氢键OHO角度接近直线；3、四个氢键指向四面体的四个顶点。然而，这三个条件只有在体相中才能同时满足。在纳米受限的环境下，这些条件中必然有一条或更多会被破坏。牺牲每一个条件的例子都存在，而对此的不同选择就可能在相近的条件下得到不同的水结构。我们通过对夹在两层疏水板中的水的分子动力学模拟，发现了一种新结构，称之为相互嵌套的五边形双层冰结构（interlocked pentagonal bilayer ice, IPBI）。它的俯视图以四个小五边形组成的大六边形为基本图样，看起来跟文献中的蹄槽式双层冰结构（Coffins bilayer ice）十分相似。但两种结构具有若干基本的区别，包括单胞中的分子数、分子在各个高度的分布、侧视图中的基本图样等。分析发现，这些区别都源自对氢键网络条件的不同取舍。IPBI牺牲了氢键的直线性，保持了氢键的四面体取向，而Coffins反之。将不同的水结构视为水分子对相近的氢键网络建构问题做出的不同解答，可以作为一种普遍性的视角。

The microscopic origin of charge-density-wave (CDW) and related phenomena in transition-metal dichalcogenides have been extensively investigated and are still hotly debated since many factors are involved in its low-energy processes, including electron correlation, electron-phonon coupling, Fermi surface nesting, metal-insulator transition, charge degree of freedom, and most probably the spin and orbital degrees of freedom. In this work, by implementing a first-principles electronic structure calculation based on WIEN2K code together with detailed analytical study, we uncover the electronic structures, magnetic and orbital properties of 1T-TaS2 monolayer, as well as bulk, we pay our special attention on the roles of lattice distortion and orbital freedom degree on the formation of CDW and related low-energy properties, a factor which many authors have seldom concerned, however, is crucial for our systematically understand various properties of 1T-TaS2. After optimizing crystal structures in bulk and monolayer 1T-TaS2, we obtain lattice parameters and atomic positions in star-of-David structure, and show that the low-temperature band structures of distorted bulk are consistent with recent angle resolved photoemission spectroscopy (ARPES) data. We further demonstrate that planar Ta form ordered orbital-density-wave (ODW) state with
dominant 5d_3z2-r2 character in central Ta, driving the monolayer into a  half-filled insulator. Meanwhile, the star-of-David distortion in monolayers stabilizes charge-density-wave order and the flat band favors ferromagnetic-density-wave order
of Ta spins with the same wavevector of ODW.  We propose that 1T-TaS2 monolayer may pave new ways to study exciton physics,
flat-band physics, and potential applications in luminescence.

探索研究具有电子学、光电子学以及自旋电子学等应用潜力的新型二维材料已成为当前物理学及材料学领域研究的热点。通过理论研究，可以预测稳定的二维结构，确定其电子结构和光学等性质，寻找调控二维材料性质的手段如：应力、电场、界面效应和功能化等。报告将结合近年来我们的第一性原理研究讨论几类稳定的二维结构，如具有狄拉克锥能带结构特征的janugraphene和chlorographene体系、d-电子金属二维体系等。探讨应力、电场及界面效应等对二维体系的电子结构及自旋电子学相关性质的调控作用，以及二维材料结构表现出的新颖物理、化学性质，如：二维材料的多体效应、量子自旋霍尔效应及多体效应量子拓扑态等。  

局域表面等离激元使得贵金属纳米材料能够有效的驱动光化学反应，其原子尺度的机制仍然是很大的挑战。利用含时密度泛函理论，我们研究了在外加飞秒激光作用下，银原子链上实时的氢分解动力学。银原子链上等离激元诱导的电荷转移到氢分子的反键轨道，最终导致氢分解，结论与实验结果一致。增加激光强度和原子链长度可以增强分解反应，这种增强效应可以归因于更强的电子响应。定量的描述实时电子响应为等离激元诱导的反应提供了基本的微观解释。

We report on first-principles calculations that clarify the structural evolution and optoelectronic properties of freestanding multilayer silicene. In use of our newly developed global optimization algorithm, we find the existence of rich dynamically stable multilayer silicene phases. A bond-angle rule is developed to explain the relative stability among them. The stable bulk Si(111) surface structures are also obtained when the Si thickness gets larger than four, showing the critical thickness for the structural evolution from monolayer Si to bulk Si. We also find that the multilayer silicene with π-bonded chain surfaces present outstanding optoelectronic properties for the solar cells and optical fiber communications due to the incorporation of sp2 hybridization in the sp3 hybridized system, which lightens an efficient way to the material design for the optoelectronic devices.

 We discover a new quantum Monte Carlo (QMC) method to solve the fermion sign
problem in interacting fermion models by employing Majorana representation of complex fermions.
We call it 'Majorana QMC' (MQMC). MQMC simulations can be performed eciently both at
nite and zero temperatures. Especially, MQMC is fermion sign free in simulating a class of spinless
fermion models on bipartite lattices at half lling and with arbitrary range of (unfrustrated) inter-
actions. Moreover, we nd a class of SU(N) fermionic models with odd N, which are sign-free in
MQMC but whose sign problem cannot be in solved in other QMC methods such as continuous-time
QMC. To the best of our knowledge, MQMC is the rst auxiliary eld QMC method to solve fermion
sign problem in spinless (more generally, odd number of species) fermion models. We conjecture
that MQMC could be applied to solve fermion sign problem in more generic fermionic models.

随着石墨烯和过渡金属硫族化物(TMDs)等二维半导体材料的兴起，新型二维半导体异质结正在成为材料科学以及光伏器件研究中的焦点。这类异质结是由不同二维半导体材料通过范德瓦尔斯力垂直堆积而成。在这种组合形成的异质结中，不仅整个系统的光电性能大大的提高，而且也出现了新奇的物理现象和器件功能。近年来，基于石墨烯和TMDs的半导体异质结弥补了石墨烯零带隙带来的应用限制，实现了具有极大电流调控的新一代垂直场效应晶体管。紧接着，以二硫化钼(MoS2)和二硫化钨(WS2)为代表的新型二维半导体异质结迅速成为材料科学领域的研究热点，并且在理论和实验上同时发现MoS2-WS2异质结具有II型的带阶排列，有效的电子空穴对分离和超快的电荷转移。基于含时密度泛函，我们研究了这种二维半导体异质结的超快电荷传输与电子-空穴对的空间分离的机理与层间堆叠方式对电荷分离的影响。研究发现：MoS2-WS2异质结中的光激发生成的电子-空穴对能在数十分秒的超短时间内发生空间分离，而且对应于不同的堆叠方式，传输的时间能相差几倍到十几倍。这对半导体材料在光伏器件，太阳能电池，光探测，光催化等过程的微观理解有这很好指导意义，并能进一步激发人们对基于TMDs二维异质结光电子器件的研究兴趣并探索其在新一代电子器件特别是太阳能电池中的巨大应用潜力。

Nonadiabatic molecular dynamics investigations on the ultrafast hole dynamics in CH₃OH/TiO₂

 

Weibin Chu¹, Wissam A. Saidi², Hrvoje Petek³ and Jin Zhao^1,3

¹Department of Physics and ICQD/Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China

²Department of Chemical Engineering, University of Pittsburgh, Pittsburgh PA 15260 USA

³Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh PA 15260 USA

 

We carried out nonadiabatic molecular dynamics investigations on the hole dynamics of CH₃OH/TiO₂ system by using the methods developed by O. V. Prezhdo et.al. We considered the molecular, half-dissociated and fully dissociated adsorption for 1ML coverage. Starting from the ground state structures, we carried ab initio MD simulations with 1 fs time step at which we first used velocity rescaling to bring the temperature of the system to 100 K and then performed a 5 ps adiabatic MD simulation in the microcanonical ensemble. The MD trajectory path obtained is then used to sample different initial conditions and perform the NAMD simulations. The hole-acceptor state is identified by finding the highest-energy adiabatic state below the valence-band maximum with the highest localization on the methanol/methoxy oxygen atoms. To simulate the NA charge dynamics, we first randomly select an initial geometry from the MD trajectory and we promote the hole from the conduction band to the hole-acceptor state. The NAMD equations are propagated to 100 fs length by solving time-dependent DFT equations. We used 100 initial configurations and report average quantities. Our NAMD results show that the hole dynamics behavior for the molecular adsorption and half-dissociated adsorption structure are quite similar. The ultrafast decay of the photo-generated hole happens within 8 fs.  However, the hole dynamics for the fully-dissociated structure is very different. It is shown that the photo-excited hole transfers back and forth between the CH₃O and the TiO₂ conduction band in a quite long time range, which suggests that CH₃O has a strong non-adiabatic coupling with the TiO₂ surface and better ability to trap the photo-generated holes.  

The ionization potential (IP) is the key to determine valence and conduction band edges of a semiconductor with respect to the vacuum level, which play an important role in many physical and chemical properties of surfaces and interfaces. In spite of its far-reaching significance, theoretical determination of IPs has not attained as much attention as that of band gaps. In this work, we investigate the performance of the GW method for the description of ionization potentials of semiconductors [1], using the all-electron GW approach implemented in the linearized augmented planewaves (LAPW) basis [2], we have shown that the consideration of the GW quasi-particle correction is necessary to obtain ionization potentials of typical semiconductors in quantitative agreement with experiment. We have made a critical comparison between two GW correction schemes, one considering the GW correction to the valence band maximum (VBM) of bulk systems, and the other assuming that the LDA/GGA gives correct band gap center (BGC) [3]. Our study shows that the VBM scheme is theoretically better founded and practically leads to closer agreement with experiment than the BGC scheme. We further found that when the LAPW basis used in GW₀ calculations is extended by additional high-energy local orbitals [4], the GW corrections to the VBM of bulk systems are significantly enhanced, which further improves the agreement of the GW-corrected IPs with experiment.

[1] H. Jiang and Y.-C. Shen, J. Chem. Phys. 139, 164114 (2013).

[2] H. Jiang et al., Computer Phys. Commun.,184, 348(2013).

[3] M. Toroker et al, Phys. Chem. Chem. Phys. 13, 16644(2011).

[4] R. Laskowski and P. Blaha, Phys. Rev. B, 85, 035132 (2012).

Engineering the vacancy states of graphene and related materials from first-principles

Mingwen Zhao

School of Physics and State Key Laboratory of Crystal Materials, Shandong University

E-mail: zmw@sdu.edu.

 

Abstract: 

Vacancy defects commonly exist in two-dimensional (2D) materials, whose effects on the electronic properties are more pronounced than in the three-dimensional (3D) counterparts. This offers additional degrees of freedom for tuning the electronic properties of 2D systems to meet the requirements of applications in various fields, ranging from spintronics to energy generation. As the most famous 2D material, graphene has been drawing considerable attention, due to its unique properties. Perfect graphene has a spin-degenerate ground state. However, by introducing vacancy defects, one can achieve stable electron spin-polarization and ferromagnetic ordering in graphene. This is related to the symmetry-breaking of the π-bonds in the vicinity of the vacancies, which can be well interpretated from first-principles calculations combined with a tight-binding (TB) model [1]. Interestingly, this mechanism still holds for other 2D materials beyond graphene, such as boron nitride and silicon carbide. The mechanism has been confirmed by our experiments [2]. The local electron states of vacancies within the band gap of semiconducting 2D materials can also be used to achieve laser saturable adsorption, which is essential for solid-state laser devices. This has been realized in MoS₂ and WS₂ nanosheets in recent experiments [3].

Additionally, there are abundant 2D porous nanostructures. Most of them have been synthesized. These porous structures have well-defined and uniformly-distributed vacancies, and thus can be regarded as vacancy lattices. The vacancy states in these periodic frameworks exhibit exotic properties that differ significantly from the randomly-generated vacancies in 2D materials. For example, apart from stable ferromagnetism, we demonstrated that the vacancy lattices have topologically nontrivial electronic structures that can be characterized by topological invariants, such as Z₂ and Chern number. By engineering the shape of the vacancies and their distribution pattern, one can obtain a Z₂ topological insulator or a Chern topological insulator with sizeable topologically nontrivial bulk gaps, which are implementable for hosting quantum spin Hall effect (QSHE) or quantum anomalous Hall effect (QAHE) at relatively high temperature [4,5].

Taking advantage of the size-tunable vacancies of these 2D materials, one can also achieve efficient gas separation. Different vacancies lead to selectivity of different gas molecule. From first-principles, we demonstrated that the abundant porous configurations of graphynes and graphitic carbon nitrides are quite promising for such applications [6].

References

[1] H. H. Xia et al., Tunable Magnetism in Carbon-Ion-Implanted Highly Oriented Pyrolytic Graphite, Adv. Mater. 20, 4679 (2008).

[2] M. Du et al., One-Step Exfoliation and Fluorination of Boron Nitride Nanosheets and a Study of Their Magnetic Properties, Angew. Chem. Int. Ed. 53, 3645-3649 (2014).

[3] A.Z. Wang, X.M. Zhang, M.W. Zhao, Topological insulator states in a honeycomb lattice of s-triazines, Nanoscale 6, 11157-11162 (2014)

[4] S.X. Wang, et al., Broadband Few-Layer MoS₂ Saturable Absorbers, Adv. Mater. 26, 3538-3544 (2014). 

[5] X.M. Zhang, A. Z. Wang, M.W. Zhao, Spin-gapless semiconducting graphitic carbon nitrides: A theoretical design from first principles, Carbon 84, 1-8 (2015).

[6] F. Li, Y.Y. Qu, M.W. Zhao, Efficient Helium Separation of graphitic Carbon Nitride Membrane, to be published

近几年，原子尺度厚的VIB族层状过渡金属二硫属化物（TMD, 化学式为MX₂: M=Mo,W; X=S, Se）已成为新兴的二维半导体材料。它们有着奇特的性质，包括单层下可见光波段的直接带隙、显著的自旋轨道耦合、超强的库仑相互作用、以及与谷自由度相关的丰富物理性质。这些二维TMD在新型电子学器件方面有着重要的应用前景从而引起了研究者们的广泛兴趣。二维TMD丰富的物理性质是由其复杂的电子结构所决定的。于是，该报告将从单层及双层TMD的能带特征、对称性、谷依赖的光选择定则、激子效应、不同自由度间的耦合、Berry相位相关属性、以及应变对能带结构的影响等多个方面介绍其电子结构。此外，该报告还将介绍目前各种近似下描述二维TMD的理论模型（包括k.p和紧束缚）并分析它们各自的特点及适用范围。

从上世纪90 年代开始发展的基于第一性原理的有效哈密顿量是研究铁电材料的一个有力理论工具。这一工具能够准确的抓住一个材料体系中最重要的自由度及其相互耦合, 而以此为基础发展出
的蒙特-卡洛(MC)和分子动力学模拟(MD)可以研究体系的静态和动态性质。 在近些年来，这一理论也在多铁材料的研究当中得到了应用。多铁性材料是一种性质非常复杂的氧化物, 使用于这一类材料的有效哈密顿量方法也相对比较复杂。 这里我们通过若干具体例子介绍有效哈密顿量方法在对这一体系研究中的应用，展示通过MC和MD模拟所获得的关于这类材料的静态和动态性能。

When a beam of light (photons) is reflected at an interface, the wave nature of photons can result in spatial shifts at the interface in the plane of incidence (longitudinal shift) and normal to the plane (transverse shift), which are referred to as the Goos-H\"{a}nchen (GH) shift and Imbert-Fedorov (IF) shift, respectively. Here, we report the GH and IF shifts in Weyl semimetals (WSMs)---a promising material harboring low energy Weyl fermions, a massless fermionic cousin of photons. Our results show that GH shift in WSMs is valley-independent which is analogous to that discovered in a 2D relativistic material---graphene. However, the IF shift has never been explored in other systems, and here we show that it is valley-dependent. Furthermore, we find that the IF shift actually originates from the topological effect of the systems. Experimentally, topological related IF shift can be utilized to characterize the Weyl semimetals and to measure the Berry curvature. The valley-dependent IF shift can also induce the valley Hall effect and provides new ways for designing valleytronic devices of high efficiency.

 

Abstract

We investigate two-dimensional electric dipole sheets in the superlattice made of BaTiO₃ and BaZrO₃ using first-principles-based Monte Carlo simulations and density functional calculations. Electric dipole domains and complex patterns are observed and complex dipole structures with various symmetries (e.g.,Pma2, Cmcm ,and Pmc2₁) are further confirmed by density functional calculations, which are found to be almost degenerate in energy with the ferroelectric ground state of the Amm2 symmetry, therefore strongly resembling magnetic sheets. More complex dipole patterns, including vortices and antivortices, are also observed, which may constitute the intermediate states that overcome the high-energy barrier of different polarization orientations previously predicted by A. I. Lebedev [Phys. Solid State 55 , 1198 (2013 )]. We also show that such system possesses large electrostrictive effects that may be technologically important.

References

[1] Z. Jiang, B. Xu, F. Li, D. Wang, and C. L. Jia, Physical Review B 91, 014105 (2015).

[2] Z. Jiang, R. Zhang, D. Wang, D. Sichuga, C. L. Jia, and L. Bellaiche, Physical Review B 89, 214113 (2014 ).

 

Te具有链状晶体结构，具有较大的自旋轨道耦合相互作用，轻带和高的能带简并度可以同时实现大的Seebeck系数和电导率。虽然单质Te具有简单的晶体结构，然而实验报道了单质Te具有低的晶格热导率，该工作通过密度泛函理论计算了Te的简谐力常数和3阶非简谐力常数，利用玻尔兹曼输运方程预测其晶格热导率。计算发现Te具有非常大的三声子的相空间，其值为接近于低热导率材料SnSe，是Si的5倍，大的三声子相空间导致了其具有较低的热导率，室温下垂直链方向约为1W/m•K。此外，Te的热导率具有较强的各向异性，沿着链方向热导率是垂直链方向的2倍，这里的各向异性的一个重要因素来自于散射几率的各向异性。由于Te中具有孤对电子，孤对电子导致了垂直于链方向具有大的非简谐力常数，进而增加垂直链方向的散射几率，减小热导率。该工作从理论上研究了孤对电子对热导率的影响，这为热电材料的材料设计提供了一定的参考。

As a big band gap semiconductor, TiO₂ have many important applications in photocatalysis. In order to investigate the principle of photocatalysis on TiO₂ surface, which is primarily determined by the excited electrons and holes, we have performed time resolved two photon photoemission (TR-2PP) and scanning tunneling microscope (STM) measurements along with first principles calculations to investigate the electronic structure and ultrafast photocatalytic dynamics on TiO₂(110) surface. Based on our investigations of H₂O and CH₃OH adsorbed on TiO₂(110) surface, we found that the wet electron states, which are bound by the dangling H atoms, provide the electron transfer pathway at the aqueous-oxide interface and it could be an important media in the photocatalysis process. Then the dynamics of the excited wet electrons were investigated and strong isotopic effect is found for CH₃OH/TiO₂ system.ΔSCF calculations indicate that it may be related to a proton coupled electron transfer process, which could be important in photocatalytic dynamics. The hole induced photochemistry is indicted in the STM measurements for H₂O/TiO₂ and CH₃OH/TiO₂. To understand this process, we performed constrained DFT to study the hole induced photochemical reaction. In parallel, we perform non-adiabatic molecular dynamics to study the ultrafast electron/hole dynamics at H₂O/TiO₂ and CH₃OH/TiO₂ interfaces.

In 2009 we proposed a theoretical model of semihydrogenated graphene sheet [Nano Lett. 2009, 9, 3867], where the semihydrogenation breaks the delocalized π bonding network of graphene, leaving the 2pz electrons in the un-hydrogenated carbon atoms unpaired, and the extended p-p interactions result in a long-range ferromagnetic coupling with a high Curie temperature. Since then the semihydrogenated graphene sheet has become a theoretical paradigm to use semihydrogenation to tune the properties of other 2D sheets such as BN, AlN, GaN, ZnO, SiC, silicene and germanene. However, the key question is how to synthesize a semihydrogenated graphene sheet. Currently it remains a huge challenge for experimentalists to remove all hydrogen atoms from one side of a graphane sheet with a large area. Here we present our simulations of the synthesis based on DFT studies

高比能量锂空气电池是未来大容量纯电动汽车潜在的动力电源技术之一，然而由于充放电动力学速率低限制了其实际性能的提升，导致其充电过电位高、循环性能差、电流密度低、电极材料不稳定、电解质分解等问题。发展高活性双功能的催化剂，提高反应速率是锂空气电池的研究热点。本报告主要是利用第一性原理热力学计算研究廉价的过渡金属氧化物、碳化物、氮化物、石墨烯等体系对充电反应的固-固界面催化作用机理，建立催化剂表面结构、界面结构、电荷转移特征、吸附能等对催化活性影响的相关规律，揭示高活性析氧反应催化剂的特征结构，通过催化剂的表面微观结构设计与晶体结构预测发展新型高活性的催化剂，改善电池电化学性能。本研究不仅解决锂空气电池中充电反应动力学速率低等基本科学问题，而且为“材料基因组”研究建立典范，为筛选高活性催化剂提供理论指导。

Multiferroic materials, which simultaneously possess two or more ferroic order parameters, have returned to the hottest topics of condensed matter physics and materials science in recent years. In particular, steering magnetism by electric fields upon interfacing   ferromagnetic and ferroelectric materials to achieve an  emergent multiferroic response bears a great potential for nano-scale devices with qualitatively new functionalities.  Here we present a systematic microscopic theory, as well as an experimental evidence, uncovering a magnon-driven strong interface magnetoelectric (ME) effect acting on the nanometer range. For Co₉₂Zr₈ film deposited on ferroelectric PMN-PT we show via ferromagnetic resonance (FMR) that at room temperature this type of linear ME interaction allows for electrical control of simultaneously the magnetization precession and its damping, both of which are key elements for magnetic switching and spintronics. The experiments unravel further an electric-field-induced negative magnetic permeability effect, pointing so to a new class of optical negative-index metamaterials based on multiferroics. Open questions and future research opportunities are discussed as well.

近年来，研究人员发现一种4d过渡金属氧化物SrTcO3具有极高的奈尔温度（1023K）。出于应用的目的，我们用基于密度泛函理论的GGA+U方法预测了双轴应变对SrTcO3的奈尔温度的影响。针对一系列平面内压缩应变和拉伸应变，对c值进行优化，并针对每种结构，计算各种磁相的总能，从而计算磁交换常数和奈尔温度。结果发现，各种应变条件下，基态磁相依然为G-型反铁磁。在拉伸应变条件下，奈尔温度随应变的增加而下降。在压缩应变条件下，奈尔温度随应变的增加先升后降：当压缩应变小于 6.6% 时，奈尔温度随压缩应变的增加而增加；当压缩应变范围值在5.5%到6.7%之间时，奈尔温度达到最大值，比相应的体材料的奈尔温度约高26.6%，该奈尔温度可通过在常用基片STO/LSAT/NGO/LAO上生长SrTcO3得到；当压缩应变大于6.7%，奈尔温度随应变的增加迅速减小。此外，我们还研究了应变对SrTcO3带隙的影响，发现在受到较小的压缩应变（不大于1.58%）时，其带隙保持直接带隙的特征，带隙值（在1.533eV~1.586eV之间）与体材料的带隙值1.61eV比较接近；当压缩应变大于1.58%，带隙随应变增大迅速减小，并且从直接带隙变为间接带隙。我们的计算结果可为SrTcO3在微型电子元器件方面的应用提供理论依据。部分相关内容已发表于Solid State Communications 219(2015)25-27. 本研究得到国家自然科学基金资助（No. 11247023）。

Since the first isolation of graphene in 2004, many two-dimensional materials and a wide variety of their properties have been extensively studied and explored. However, ferroelectricity has barely been reported in such materials. In this talk, I will present our recent prediction of a class of two-dimensional van der Waals ferroelectric materials, represented by layered In₂Se₃. Based on first-principles density functional theory calculations, we discover a new structural model of a single quintuple layer In₂Se₃, which is not directly derived from any known crystal structures. This new structure features the absence of symmetry along the perpendicular direction to the In₂Se₃ layer, leading to a spontaneous out-of-plane electric polarization. The orientation of the electrical dipole can be reversed through a virtually lateral shift of the middle Se layer. The electronic properties of such materials will be discussed in this talk. The finding of the two-dimensional ferroelectric materials will expand the family of the two-dimensional materials with ferroelectricity and offers new opportunities to integrate ferroelectricity in van der Waals heterstructures for practical device applications.

From graphene to topological insulator, Dirac material continues to attract attention of researchers in various fields because of its potential to develop new electronic devices. However, the Dirac material is rare in reality and suffers from the difficulty to incorporate into the current semiconducting industry. In this talk, I will introduce our recent investigations on a distinct kind of Dirac material characterized by transition-metal d electrons, namely, metallosine, composed of transition-metal intercalated epitaxial graphene on SiC(0001). The formation, properties, modulation and stability of the massless Dirac fermions will be discussed. In comparison with graphene, the spin and valley degeneracy can be lifted spontaneously in the d-character Dirac materials, which thus hold the promise for the low-power electronic and spintronic applications.

Methods of numerical renomalization group have been frequently used in the studies of quantum many-body systems in recent years. In particular, the strong-disorder renormalization group method (SDRG), with the concept of combining the strongest coupling terms repeatedly in order to reduce the degrees of freedom, is a powerful method in studying the disordered systems. On the other hand, the method of representing the operators in a many-body system in the matrix form, which is called the matrix product operator (MPO), makes us easy to do the computations. In our study, the SDRG method, combined with the MPO representation, is applied to the 1-D random antiferromagnetic Heisenberg chain and the 1-D random transverse Ising model to derive the ground state behaviors.  The ground state energies and correlation functions are calculated.

会议主题：计算凝聚态物理

过渡金属吸附Pc片层结构的磁性及磁晶各向异性研究

朱国俊   曹觉先

湘潭大学物理与光电工程学院

近年来，为了满足人们对高密度磁存储材料的需求，越来越多的科研工作者关注纳米磁性材料领域。提高磁存储材料存储密度的一个核心问题就是如何去提高记录单元的磁晶各向异性能（MAE），即提高磁化方向翻转的势垒从而防止热扰动导致信息的丢失。众所周知，金属原子吸附在低维材料上可以引入更大的磁晶各向异性能[1]。已经有大量的工作致力于寻找合适的基底材料来吸附金属原子从而获得稳定的结构和可观的磁晶各向异性能。有研究发现，过渡金属原子对吸附到苯环或者带有缺陷的石墨烯上时结构非常稳定，并且可以保持较大的磁晶各向异性能[2]。但是，实验上可控的在石墨烯上形成均匀有序的空位缺陷是一个巨大的挑战。Phthalocyanine (Pc)分子在其中心位置具有天然的孔洞，可以稳定吸附金属原子形成(MPc)，并且部分3d过渡金属吸附Pc分子中心形成的Metal-Pc分子已表现出一定的磁晶各向异性能[3]。同时实验上已经在Au(111)和Ag(111)表面上成功制备出均匀有序的Fe-Phthalocyanine(FePc)片层结构[4]。因此片层结构的Pc是一种很有潜力的衬底材料。

运用基于密度泛函的第一性原理方法，我们对部分过渡金属(Cr, Mn, Fe, Co, Mo, Tc, Ru, Rh, W, Re, Os, Ir)吸附在Phthalocyanine片的中心孔洞位置形成的MPC片层结构进行了磁性和磁晶各向异性的研究，尝试了使用不同的应变对RePc的MAE进行调控。研究结果表明，在同周期过渡金属吸附时，磁矩随着价电子数增加表现出近似的线性变化。同时，在同主族的金属元素吸附时，通过d轨道的分波态密度图发现，金属原子具有相似的能级分布，并且磁晶各向异性能随费米面的移动具有相近的变化趋势。在所计算的12个结构中，RePc表现出高达20meV的垂直方向的磁晶各向异性能。当对RePc施加-2%至2%应变进行调控的时，RePc的磁晶各向异性能对应变表现出很好的线性响应。研究结果表示，片层MPc具有成为新一代存储材料的潜力。

致谢：本工作得到国家自然科学基金（批准号：11074212, 11204259, 11374252,11474245）和教育部新世纪优秀人才（编号：NCET-12-0722）资助。

参考文献：

[1] S.R. P. Gambardella, M. Veronese, S. S. Dhesi, C. Grazioli, A. Dallmeyer, I. Cabria, R. Zeller, P. H. Dederichs, K. Kern, C. Carbone, and H. Brune, Giant Magnetic Anisotropy of Single Cobalt Atoms and Nanoparticles, Science, 300 (2003) 1130-1133.

[2] H.C. Kandpal, K. Koepernik, M. Richter, Strong magnetic anisotropy of chemically bound Co dimers in a graphene sheet, Physical Review B, 86 (2012).

[3] J. Wang, Y. Shi, J. Cao, R. Wu, Magnetization and magnetic anisotropy of metallophthalocyanine molecules from the first principles calculations, Applied Physics Letters, 94 (2009) 122502.

[4] M. Abel, S. Clair, O. Ourdjini, M. Mossoyan, L. Porte, Single layer of polymeric Fe-phthalocyanine: an organometallic sheet on metal and thin insulating film, Journal of the American Chemical Society, 133 (2011) 1203-1205.

In recent years, the perovskite solar cells based on organic-inorganic hybrid have attracted broad attention in the fields of material research as light absorbers, owing to the very rapid development in photovoltaic performance. Since the perovskite solar cells were first reported with a PCE of 3.8% in 2009, the PCE of them has increased rapidly. In 2014, a PCE of 19.3% was demonstrated. However there has been little study on the stability of perovskite solar cells. The degradation of perovskite soalr cells has been big problem for the successful commercial application. In order to improve the stability of the perovskite solar cells, the mechanism of the degradation should be basically understood via thorough study. In this letter, we choose MAPbX₃ (MA=CH₃NH_3, X=I, Br, Cl) as an example and use first-principle calculation to systematically study the degradation of perovskite solar cells in a humid environment.

E-mail  zhaoyj@scut.edu.cn

由于在诸如发光二极管、激光二极管和高频晶体管等光电器件和微电子器件方面的潜在应用，氮化铟以及第三族的其它氮化物在实验和理论研究方面都引起了极大的关注。最近，陈卓研究组的研究人员采用SVSC方法 (solution- and vapor-phase methods under silica shell confinement)制备了氮化铟纳米晶[1]，发现随着包覆在氧化铟纳米颗粒外的二氧化硅壳层的厚度从厚变薄，氮化铟纳米颗粒呈现出从纯四方闪锌矿相到六角纤锌矿、四方闪锌矿的混合相，再到纯六角纤锌矿相的转变。在排除动力学和热力学等因素对氮化铟相变的影响过后，我们通过第一性原理计算方法，详细阐述了二氧化硅外壳引入的氮空穴浓度与六角和四方相的稳定性关系，发现二氧化硅外壳引入的化学反应计量数配比不均是导致形成亚稳相的原因。明确这两个相共存和转变的原因和机理对于实现制备过程中的精确控制有重要的意义。

1.             Chen Z, Li Y, Cao C, et al. Large-Scale Cubic InN Nanocrystals by a Combined Solution-and Vapor-Phase Method under Silica Confinement[J]. Journal of the American Chemical Society, 2012, 134(2): 780-783.

会议主题：计算凝聚态物理

180°畴壁对铁电薄膜漏电流影响的第一性原理研究

江小蔚   曹觉先

湘潭大学   物理与光电工程学院

    铁电材料由于具有光电效应、压电效应、介电效应和铁电效应等优良的性质而被广泛的用于各种电子器件当中，特别是应用于铁电存储器(FeRAM)中[1]。FeRAM具有多、快、好、省的特点，以及优良的抗辐射性能。它是已经得到广泛应用的新一代存储器，也是铁电材料发展的动力[2]。近年来，随着科技信息技术的快速发展，铁电失效成为了阻碍FeRAM进一步发展的重要因素。而随着器件的尺寸越来越小，当铁电薄膜厚度接近几纳米时，漏电流成为了引起铁电失效的关键因素也决定了器件完整性和可靠性[3]。影响漏电流的因素有很多，如前人研究提出的：工艺温度、缺陷浓度、界面等[4]。但是畴壁在铁电材料中比缺陷等因素更普遍存在，它受许多的外部因素和内部因素的影响，所以研究畴壁对漏电流的影响对解决铁电失效问题有重要的作用，并且也对铁电材料的应用和发展具有重大意义[5]。

    我们基于密度泛函理论的第一性原理并结合非平衡格林函数，探讨了180°畴壁对PbTiO3铁电薄膜漏电流的影响规律。研究表明，180°畴壁使得铁电薄膜的漏电流大大增强。同时，通过比较在不同终端下180°畴壁对铁电薄膜漏电流影响的大小，我们发现TiO端下畴壁对漏电流的影响比PbO端下大。通过分析不同终端下单畴和180°畴壁结构的透射系数T(E)和局域态密度(LDOS)发现：180°畴壁引入了额外的输运通道使得T(E)大大增强，尤其是在费米面附近，并且TiO端下单畴和180°畴壁的T(E)变化比PbO端下大；我们还分析了这几种体系的DOS发现：畴壁处的带隙相对于单畴的带隙要小，TiO端下单畴和180°畴壁的带隙变化比PbO端下要大。这些计算结果进一步分析了180°畴壁对漏电流影响的原因。本研究为提高FeRAM的性能和铁电薄膜器件的发展提供了理论上的指引。

致谢：本工作得到国家自然科学基金（批准号：11074212, 11204259, 11374252,11474245）和教育部新世纪优秀人才（编号：NCET-12-0722）资助。

参考文献：

[1] E. Y. Tsymbal and H. Kohlstedt, Science 313, 181 (2006).

[2] J. F. Scott, Applications of Modern Ferroelectrics, Science 315, 954-959(2007) 

[3] J. P. Velev, C.-G. Duan, K. D. Belashchenko, S. Jaswal, and E. Y. Tsymbal, Physical review letters 98, 137201 (2007).

[4] X. Luo, B. Wang, and Y. Zheng, ACS nano 5, 1649 (2011).

[5] H. Béa and P. Paruch, Nature materials 8, 168 (2009).

虽然实验上发现As掺ZnO具有p型导电性，但是其形成的机理直到最近都还在争论。第一性原理计算表明，As_Zn-2V_Zn在ZnO体内为受主缺陷对，计算的电离能和实验值吻合。但Van de Walle指出缺陷之间的束缚能不足以抵消V_Zn的形成能。最近的Hybrid计算表明As_Zn-2V_Zn的电离能很高，不可能是As掺ZnOp型导电性的起源。因此，As掺ZnO为什么具有p型导电性，目前仍然不清楚。本项目通过计算As在ZnO晶界的分凝，发现晶界处V_Zn的形成能只有0.15eV。同时，As_Zn-V_Zn在晶界处为受主缺陷对，其束缚能为0.15eV，和实验值吻合，可以在晶界处稳定存在。由于缺陷和杂质在晶界处富集，杂质，缺陷相互作用，形成界面能带。类似于共掺杂，界面能带形成新的价带顶，As-V_Zn受主缺陷对的电离能只有0.17eV，和实验值吻合。因此，我们认为晶界处As_Zn-V_Zn是As掺ZnOp型导电性形成的原因。

Molecular dynamics simulations are performed to study the high-temperature (T) behavior of monolayer graphyne and graphdiyne (MGY and MGDY). During the melting process, MGY and MGDY undergo three continuous phase transitions: (i) transformation into the initial amorphous graphene phase (AGP) starting from 2800 and 2500 K for MGY and MGDY, respectively, where broken bonds and large holes appear; (ii) transformation into a perfect AGP with fewer defects through a regularization process until T reaches 3650 K. This perfect AGP will remain relatively stable until T reaches approximately 5000 K; (iii) the AGP will then gradually transform into a nearly fluid state for T higher than 5000 K. Even prior to the first phase transition, some defects, such as 5,7,8-membered rings start appearing at 1700 and 1200 K for MGY and MGDY, respectively. We find that in addition to the defects derived from severe thermal vibration, the sharp contraction and accompanying stretch of MGY and MGDY are decisive in the formation of the initial AGP. In contrast to the prediction of the Kosterlitz-Thouless-HalperinNelson-Young theory for the melting of two-dimensional materials, we do not observe a hexatic phase but rather an AGP as an intermediate phase.

We investigate the electronic structure and magnetism of semi-Heusler alloys CoCrP and CoCrAs using the full-potential linearized augmented plane wave method. The calculations reveal that CoCrP and CoCrAs are half-metallic ferromagnets with the same magnetic moment of 2.00 µ_B per formula unit. Both alloys have large half-metallic gaps (up to 0.50 eV) and wide band gaps (above 1 eV). The half-metallicity of CoCrP and CoCrAs can be retained even when their lattice constants are changed by -4.8 to 6.6% and -7.7 to 4.5%, respectively. The two alloys show great promise in spintronic applications.We investigate the electronic structure and magnetism of semi-Heusler alloys CoCrP and CoCrAs using the full-potential linearized augmented plane wave method. The calculations reveal that CoCrP and CoCrAs are half-metallic ferromagnets with the same magnetic moment of 2.00 µ_B per formula unit. Both alloys have large half-metallic gaps (up to 0.50 eV) and wide band gaps (above 1 eV). The half-metallicity of CoCrP and CoCrAs can be retained even when their lattice constants are changed by -4.8 to 6.6% and -7.7 to 4.5%, respectively. The two alloys show great promise in spintronic applications.

A method based on the particle swarm optimization (PSO) algorithm is presented to design quasi-two-dimensional (Q2D) materials. With this development, various single-layer and bi-layer materials in C, Si, Ge, Sn, and Pb were predicted. A new Si bi-layer structure is found to have a much-favored energy than the previously widely accepted configuration. Both single-layer and bi-layer Si materials have small band gaps, limiting their usages in optoelectronic applications. Hydrogenation has therefore been used to tune the electronic and optical properties of Si layers. We discover two hydrogenated materials of layered Si₈H₂ and Si₆H₂ possessing quasi-direct band gaps of 0.75 eV and 1.59 eV, respectively. Their potential applications for light emitting diode and photovoltaics are proposed and discussed. Our study opened up the possibility of hydrogenated Si layered materials as next-generation optoelectronic devices.

We propose that a polar-nonpolar transition-metal oxide heterostructure can be used as a highly-efficient photocatalyst. This is demonstrated using a SrTiO3/LaAlO3/SrTiO3 sandwich-like heterostructure with a photocatalytic activity in the near-infrared region. The effect of the polar nature of LaAlO3 is two fold. First, the induced electrostatic field accelerates the photo-generated electrons and holes into opposite directions and minimizes their recombination rates. Hence, the reduction and oxidation reactions can be instigated at the SrTiO3 layers located on opposite sides of the heterostructure. Second, the electric field reduces the band gap of the system making it photoactive in the infrared regime. We show also that charge separation can also be enhanced using compressive strain engineering that creates a ferroelectric instability in STO. The proposed setup is ideal for tandem photocatalysts especially when combined using photoactive polar materials such as LaVO3 or GdTiO3 .  We haven’t got any new results related to LVO or GdTiO3. Shall we just say that this design shall work for all these kinds of polar-nonpolar oxide heterostructures? In the discussion we can mention LVO or GdTiO3 as possibilities.

We investigated toxic formaldehyde (HCHO) gas decomposing on stoichiometric rutile (1 1 0) surface via DFT calculations. Recent experimental results demonstrated that HCHO would decompose to carbon monoxide (CO) and water (H₂O) under ultraviolet light illumination on rutile (110) surface, and CO could be converted to carbon dioxide (CO₂) during following procedures. Through DFT calculations, we found three stable adsorption configurations of HCHO on stoichiometric rutile (110) surface; among them the bidentate adsorption structure is the most stable one by forming O-Ti_5c and C-O_b bonds with substrate. Further we searched reaction paths for adsorbed HCHO turning into CO catalyzed by TiO₂ via nudged elastic band (NEB) method; energy barriers of 2 eV for direct decomposition and a lower one of 1.7 eV for indirect decomposition were found for HCHO decomposing to CO. These findings revealed the decomposition mechanism of HCHO to CO on stoichiometric rutile (1 1 0) surface and enriched the knowledge of organic reaction under catalyzed by metal oxides.

Theoretical Investigation on the Structural Stability and Electronics Properties of SiGe Nanocrystals

Xiaolin Deng and Xiaobao Yang
Department of Physics, South China University of Technology, Guangzhou 510640, P.R. China

Abstract: We are studying the structural stability and electronic property of SiGe nanocrystals. By calculating the partition function combined with the first principle calculation, we obtain the probability of SiGe Nanocrystals with various Si/Ge distributions under various different temperatures. In addition, we will study the electronic properties (including band gap, HOMOs(highest occupied molecular orbital) and LUMOs(Lowest unoccupied molecular orbital)) of these most stable structure.

Key words: First principle, partition function, electronic properties

scxbyang@scut.edu.cn

We investigate by going beyond the quasiharmonic approximation the phonon dispersions of iron at the gamma(fcc)-delta(bcc) phase transition using the self-consistent ab initio lattice dynamical method in combination with effective magnetic force calculations. In the high-temperature delta phase anharmonic phonon-phonon interactions are found to play a crucial role in determining the mechanical stability of the open bcc lattice. Compared with delta iron, the anharmoic lattice effect is much smaller in the closed-packed fcc gamma phase where magnetism has a strong effect on its phonon behavior. Based on the calculated temperature-dependent phonon dispersions for delta and gamma iron, which are in good agreement with recent experimental data, our thermodynamic analysis demonstrates the vibrational entropy driven nature of the gamma-delta structural phase transition in paramagnetic iron.

Theoretical studies of the luminescence properties in ScVO₄ doped with Bi³⁺

Haishan Zhang and Xiaobao Yang*

Department of Physics, South China University of Technology, Guangzhou 510640, P.R. China

 

Abstract:Scandium vanadate (ScVO₄) undoped and doped with Bi³⁺ will induce a blue emission and a red emission under the excitation of UV light, respectively. The luminescence mechanisms are based on the energy level schemes that combine the host and the dopant states .So, understanding the formation of various defects in the process of Bi³⁺ doping ScVO₄ is quite necessary. According to the calculated formation energy of various possible defects by density functional theory, the substitution of Sc atoms by Bi atoms would be stable in room temperature. With the increase of temperature, an oxygen atom near the Bi atom will lose more easily, which thus form the complex-defect Bi_Sc+V_O. Further analysis of the projected density of states(PDOS) shows that the dominant defect Bi_Sc only induces a slight upward shifting of the valence band maximum(VBM). In addition, the complex-defect Bi_Sc+V_O has a occupied defect level near the Fermi level that mainly ascribe to Bi³⁺ ions, the distance from the bottom of conduction band(CBM) to the defect level is basically consistent with the red emission position (~635nm). Thus electrons from the CBM to the impurity level will give rise to the emergence of the red light.

*Email：scxbyang@scut.edu.cn

   热电转换材料作为一种新能源技术可以实现将废热转换成电能，具有广泛的应用前景。而当前，热电材料的实际应用受限于其极低的品质因子，造成热电转换效率不足10%。一方面，寻找高品质因子的热电材料成为重要研究课题；另一方面，通过一定途径来提高功率因子或者降低热导率可以进一步优化现有热电材料的品质因子[1,2]。

   最近，赵等人通过实验在IV-VI族层状化合物SnSe中发现了极低的热导率和极高的品质因子[3]。而与SnSe具有相同结构的IV-VI族化合物还有GeS, GeSe, SnS。运用第一性原理和玻尔兹曼输运方程，我们计算并比较了这四种化合物的热电性质。其中SnSe的计算结果与实验比较符合，并且发现GeS, GeSe, SnS完全具有可以和SnSe相比拟的热电性质。较高的功率因子和极低的热导率使得这四种化合物成为潜在的新型块体热电材料[4]。

   此外，能带工程可以实现提高塞贝克系数的同时不降低电导率，进而可以提高功率因子来实现对品质因子的优化。最近，Bilc等人基于块体半霍伊斯勒半导体，通过调控能带结构中高度取向的电子态，将功率因子提高了4到5倍[5]。同时，我们发现传统热电材料PbTe在L点具有能带简并的趋势。采用与Bilc类似的原子替换方法，实现了对L点能带的有效调控，能带简并程度增强。最终将n型功率因子提高了3到8倍，进一步证明了能带工程是优化热电性能的有效途径之一[6]。

[1] J. Q. He, M. G. Kanatzidis and V. P. Dravid, Mater. Today. 16, 1369 (2013).

[2] G. Ding, G. Y. Gao and K. L. Yao, J. Phys. D: Appl. Phys. 48, 235302 (2015). 

[3] L. Zhao, S. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V. P. Dravid and M. G. Kanatzidis, Nature. 508, 373 (2014). 

[4] G. Ding, G. Y. Gao and K. L. Yao, Sci. Rep. 5, 9567 (2015).

[5] D. I. Bilc, G. Hautier, D. Waroquiers, G. M Rignanese and P. Ghosez, Phys. Rev. Lett. 114, 136601 (2015).

[6] G. Ding, G. Y. Gao, J. Li and K. L. Yao. (unpublished)

 

 

The abundant crystal phases in III−V semiconductor nanowires lead to their sidewalls featured with different crystal facets. Owing to the large surface-to-volume ratio, the crystal facet effect becomes a key factor for electronic and optical properties of III−V semiconductor nanowires. The facet and crystal phase effects on the structural stability and electronic properties of III-V (including InP and GaP) nanowires (NWs) are investigated by using first-principles calculation within density-function theory. For the wurtzite InP NWs, the surface-energy calculation suggests that side-facet structures of InP NWs are unreconstructed due to the fact that the low-index facets with paired In-P dimers satisfy the electron counting rule. The size dependence of NW band gap indicates that the band gap (E_g) of uniform-sized InP NWs with different side-facets follows the trend, E_g-{1120}>E_{g-{1100}-{1120}}>E_g-{1100} , when the NW diameter is larger than 3nm but a reverse trend is found in the smaller sized NWs (d<3 nm). For the GaP NWs, The stability mechanism of GaP nanowires depends on the competition between the crystal phase and facet effects: the former dominates the stability of larger-sized nanowires, but the stability of ultrathin nanowires is mainly determined by the latter. Likewise, electronic properties of GaP nanowires, including band gap values and the band structure characteristic, are sensitive to not only the nanowire size but also the crystal phase.

 

金属团簇相比于块体材料表现出很多新颖的物理和化学性质，但是关于金属团簇的磁学性质随吸附原子的变化规律仍然存在有较多的争议。我们运用第一性原理方法对这个问题进行了系统的理论研究，计算结果表明团簇的磁学性质随其组分和吸附原子的多少变化呈现出不同的规律。当团簇较小的时候，其磁矩随吸附原子个数增大；但是当团簇较大，组分较复杂时，其相应的磁矩随吸附原子的个数呈现出较复杂的变化。我们从吸附原子引起的电子结构和电荷转移等方面对这些现象给予了理论解释，我们的结果对以后的实验具有一定的指导意义。

Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of practical interest for the development of novel memory devices that allow for electrical writing and non-destructive magnetic readout operation. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. BiFeO₃ is the most heavily investigated single-phase multiferroic to date due to the coexistence of its magnetic order and ferroelectric order at room temperature. However, there is no net magnetic moment in the cycloidal (antiferromagnetic-like) magnetic state of bulk BiFeO₃, which severely limits its realistic applications in electric field controlled memory devices. Here, we predict that LiNbO₃-type Zn₂FeOsO₆ is a new multiferroic with properties superior to BiFeO₃. First, there are strong ferroelectricity and strong ferrimagnetism at room temperature in Zn₂FeOsO₆. Second, the easy-plane of the spontaneous magnetization can be switched by an external electric field, evidencing the strong magnetoelectric coupling existing in this system. Our results suggest that ferrimagnetic 3d-5d LiNbO₃-type material may therefore be used to achieve voltage control of magnetism in future memory devices.

采用第一性原理计算方法, 研究了四方MoSi 2 薄膜的电子性质. 计算结果表明, 各种厚度的薄膜都是金属性的, 并且随着厚度的增加, 其态密度与能带结构都逐渐趋近于MoSi 2 块体的特性. 通过对MoSi 2 薄膜磁性的分析, 发现三个原子层厚的薄膜具有磁性, 其原胞净磁矩为0.33 µ B ; 而当薄膜的厚度大于三个原子层时,薄膜不具有磁性. 此外, 进一步对单侧加氢饱和以及双侧加氢饱和结构下三原子层MoSi 2 薄膜的电子性质进行了研究, 发现单侧加氢饱和的三原子层MoSi 2 薄膜具有磁性, 其原胞净磁矩为0.26 µ B , 而双侧加氢饱和三原子层MoSi 2 薄膜是非磁性的. 双侧未饱和与单侧加氢饱和的三原子层MoSi 2 薄膜的自旋极化率分别为30%和33%. 这些研究结果表明, 三原子层厚的MoSi 2 超薄薄膜在悬空或者生长于基底之上时具有金属磁性, 预示着它在纳米电子学和自旋电子学器件等方面都有潜在的应用前景.

Ya-Ting Wang ¹, Yu-Jun Zhao^1,2 and Xiao-Bao Yang^{1, 2*}

¹Department of Physics, South China University of Technology, Guangzhou 510640, People’s Republic of China

² Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510640, P. R. China

     We have performed a systematic investigation on structural stabilities of the hydrogenated cubic silicon carbide nanocrystals (H-SiCNCs), based on the bond energy model and the first-principles calculations. By screening possible candidates with the Wang-Landau method and the convex analysis, we have determined the stable H-SiCNCs, which are in line with the first-principles calculations. The configurations of H-SiCNCs are dominated by the hydrogen and carbon chemical potentials according to the phase diagram, and there are structural transitions with the increasing size from tetrahedron, hexahedron, to octahedron, where the stable H-SiCNCs contain more carbon atoms than silicon with the chemical potential’s constraint of bulk silicon carbide. The energy gaps of tetrahedral H-SiCNCs are larger than that of octahedral ones at similar sizes, and in hexagonal ones there is a charge separation for the highest occupied molecular orbitals and lowest unoccupied molecular orbitals. Our finding provides a practical avenue to tune the electronic properties of H-SiCNCs, by modulating their shapes and surface morphologies with the chemical potential control of C/Si and H passivants.

 

^* E-mail address: scxbyang@scut.edu.cn

By performing first-principle quantum transport calculations, we studied the spin-dependent transport properties of bilayer graphene nanoribbons with different stacking and different magnetic states. Caused by the symmetry matching of spin subbands of the left and right electrodes, the negative differential resistance effects are observed in both AA stacking and AB stacking bilayer graphene nanoribbons. An obvious spin-polarization of current is also obtained under interlayer anti-ferromagnetic and interlayer ferromagnetic states. When the bias is zero, the AB stacking bilayer graphene nanoribbons show a typical property as semi-half-metal.

By first-principles calculations, we find that BiCrO3 can stabilize in three structures when relaxing from different experimental crystal lattice. The experimental undetermined structure is identified to be non-centrosymmetric R3c which permits the spontaneous ferroelectric polarization. The atomic positions within the unit cell were calculated using the GGA, GGA + U, LDA and LDA + U exchange–correlation energy density functionals, then the lattice constants along with the atomic positions were relaxed to find the ground state structure. All the calculations indicate that the Pnma structure is lower in energy than R3c and C2/c.

采用基于密度泛函理论（DFT）的第一性原理方法研究了金属多层膜的磁学性质。通过系统地对具有不同结构和组分的金属多层膜进行研究，发现不仅金属多层膜的磁各向异性能的数值随组分和结构发生了很大的改变，而且其相应的磁化方向也发生了很大的改变。在此基础上，提出了应力也可以用来对金属多层膜的磁学性质进行调控的方法

Utilizing first-principles theory, we demonstrate that half-metallicity can be realized in a junction composed of non-magnetic armchair graphene nanoribbon (AGNR) and ferromagnetic Ni electrodes. The half-metallic property originates from the AGNR energy gap of up spin located at the Fermi energy, while large states are generated for down spin. By altering the interlayer distance and the contact area, namely, the strength of AGNR-Ni interaction, the efficiency of spin filter becomes lower, since the energy gap moves away from the Fermi energy with the variation of charge transfer intensity. 

    氢能源是一种绿色环保、能量密度高、储量丰富的可再生能源，最有希望替代化石能源。在氢能源开发和利用过程中，储存和运输氢是其重要环节，所以研究储氢材料显得尤为重要。金属修饰的二维纳米结构材料因其物理化学性质稳定、表面积大、质量轻等优点常被用于研究其储氢性能。但金属在石墨烯[1]、C3N4片[2]、BN片[3]等纳米结构上容易发生团聚，进而影响其储氢量。石墨炔是石墨烯的同素异形体，由sp 和sp2杂化的碳原子组成。sp杂化的碳有较多悬挂键从而增强金属与衬底材料的作用，阻止金属在表面发生团聚。2010年中科院化学所的李国兴等人[4]利用铜片催化六炔基苯发生偶联反应，成功制备了大面积的石墨炔（graphdiyne）。这一研究成果将为大面积石墨炔薄膜在储氢材料上的应用开辟一条道路。

    我们基于密度泛函理论的第一性原理计算，系统分析了锂原子修饰6,6,12-石墨炔复合材料的储氢性能。研究表明，锂原子稳定地分散在石墨炔片上，没有形成锂团簇，其平均吸附能为 -0.361eV。在复合材料上添加不同数量的氢分子，充分考虑它们的各种吸附位，结果表明有30对氢分子稳定地吸附在锂原子周围，储氢量高达 19.3wt%，氢分子的平均吸附能为 -0.230eV。通过分析锂与石墨炔、氢分子与衬底的PDOS图和差分电荷密度图，发现锂的部分电荷转移到石墨炔上，从而产生较强的静电场，在静电场的作用下，氢分子发生了极化，其电荷由远离锂的一端转移到近锂端，分布在氢与锂之间，氢和锂通过静电作用相互吸引。同时氢分子之间形成的大分子结构进一步降低了它们的吸附能。我们的计算结果预示锂修饰的6,6,12-石墨炔片是一种潜在的具有高储氢量的储氢材料，为实验制备储氢材料提供一种理论参考。同时我们研究了四种金属（Li、Ca、Sc、Ti）修饰炔管的储氢性能，发现由于炔管的卷曲效应，使得金属与炔管的作用强于与γ-石墨炔片的作用，解决了金属在γ-石墨炔片上团聚的问题。

    致谢：本工作得到国家自然科学基金（批准号：11074212, 11204259, 11374252,11474245）和教育部新世纪优秀人才（编号：NCET-12-0722）资助。

参考文献：

1. G. Kim, S. H. Jhi, Phys. Rev. B 78, 085408 (2008).
2. Y. Zhang, H. Sun, C. Chen, Phys. Lett. A 373, 2778 (2009).
3. Q. Deng, L. Zhao, Y. Luo, M. Zhang, Li. Zhao, Y. Zhao, Nanoscale 3, 4824 (2011).
4. G. Li, Y. Li, H. Liu, Y. Guo, Y. Li, D. Zhu, Chem. Commun., 46, 3256 (2010).

Spin-S singlet-product states on spatially anisotropic lattices

 

Kevin S. D. Beach (University of Mississippi)

 

I will discuss a family of singlet-product wave functions that encompass both resonating-valence-bond (RVB) and Affleck-Lieb-Kennedy-Tasaki (AKLT) states for systems of arbitrary spin S. In the case where the valence bonds respect the bipartite structure of the lattice, it is possible to evaluate numerically any expectation value of interest: e.g., magnetism, dimer order, and even the so-called strange correlator. States made up of nearest-neighbour (NN) singlets only are often invoked as prototype ground states for various frustrated models in two dimensions (2D) because of their spin-liquid character. In three dimensions (3D), however, such states are generally not featureless, and their tendency is toward robust antiferromagnetic order. I present results for layered systems, in which it is straightforward to interpolate between the 2D and 3D cases and to identify and characterize the transitions between ordered and disordered phases.

Carrier relaxation and transport in photo-doped Mott insulators

 

Philipp Werner (University of Fribourg)

 

We use the nonequilibrium generalization of dynamical mean field theory [1] to study the relaxation and transport of photo-excited carriers in Mott insulators. In small gap insulators, impact ionization processes can lead to a rapid increase of doublon-hole pairs [2], which may be relevant for the harvesting of high-energy photons in Mott insulating solar cells [3]. A competing effect in systems with strong antiferromagnetic correlations is the loss of kinetic energy by spin-flip scattering [4]. An efficient local dissipation mechanism however turns out to be important for the mobility and separation of photo-carriers in strong electric fields [5].

 

[1] H. Aoki, N. Tsuji, M. Eckstein, M. Kollar, T. Oka, and P. Werner, Rev. Mod. Phys. 86, 779 (2014)

[2] P. Werner, K. Held and M. Eckstein, Phys. Rev. B 90, 235102 (2014)

[3] E. Assmann, P. Blaha, R. Laskowski, K. Held, S. Okamoto, and G. Sangiovanni, Phys. Rev. Lett. 110, 078701 (2013)

[4]  M. Eckstein and P. Werner, arXiv:1410.3956

[5]  M. Eckstein and P. Werner, Phys. Rev. Lett. 113, 076405 (2014)

 

Entanglement spectra from fermion quantum Monte Carlo simulations

 

Fakher F. Assaad (University of Wuerzburg)

 

 Topological phases of matter can eciently be detected by analyzing the entanglement spectrum. In this talk, I will show how compute equal time and time displaced correlation function of the the entanglement Hamiltonian of a real space bipartition [1; 2] with fermion quantum Monte Carlo methods. To test the method we will consider the Kane Mele Hubbard model [3; 4] and show that we can detect the interaction driven quantum phase transition from the correlated topological insulator to quantum antiferromagnetic by analyzing the entanglement spectrum.

 

[1] F.F. Assaad, T.C. Lang, and F. ParisenToldin, Phys. Rev. B 89, 125121, (2014).

[2] F. F. Assaad, Phys. Rev. B 91, 125146 (2015).

[3] M. Hohenadler, Z. Y. Meng, T. C. Lang, S. Wessel, A. Muramatsu, and F. F. Assaad, Phys.

Rev. B 85, 115132 (2012).

[4] M. Hohenadler, F. Parisen Toldin, I. F. Herbut, and F. F. Assaad, Phys. Rev. B 90, 085146

(2014).

 

Raising superconducting transition temperature by lifting the σ-bonding bands to the Fermi level

 

Zhong-Yi LU (卢仲毅)

Department of Physics, Renmin University of China, Beijing 100872, China

中国人民大学物理系

摘要：

Raising superconducting transition temperature (T_c) is an important task of fundamental research of superconductivity. It is also a prerequisite for the large scale application of superconductors. In the absence of microscopic mechanism of high-T_c superconductivity, the conventional approach for raising T_c is either to apply high pressure to a material which has potential to become superconducting or to push it close to an antiferromagnetic or other quantum instability point by doping. Here we introduce another approach that can be used in the search of new superconductors: it is to raise Tc by lifting the σ-bonding bands to the Fermi level, namely by metalizing σ-bonding elections. This approach can increase the probability of finding a new high-Tc superconductor because the coupling of σ-bonding electrons with phonons is generally strong and the superconducting transition induced by this interaction can happen at relatively high temperature. After elucidating the underlying mechanism, we discuss a number of schemes to metalize σ-bonding electrons, and present our recent prediction for the crystal and electronic structures of two potentially high-Tc superconductors Li₂B₃C and Li₃B₄C₂ with Tc higher than 50 K, based on the first-principles electronic structure calculations in the framework of the Eliashberg equations. [1][2]

 

[1]M. Gao, Z.-Y. Lu, and T. Xiang, Phys. Rev. B 91, 045132 (2015);

[2]高淼，卢仲毅，向涛，即将发表在《物理》。

Quench dynamics with the time-dependent DMRG

 

Salvatore R. Manmana (Georg-August-University Göttingen)

 

The density matrix renormalization group (DMRG) and related matrix product state (MPS) approaches are important numerical techniques for the investigation of ground state properties and of the dynamics of low-dimensional quantum many body systems. In my talk, I will introduce the basic concepts of MPS and DMRG approaches for nonequilibrium situations. Using these techniques, I will show various results obtained for the time evolution of 1D fermionic systems following a so-called quantum quench in which one of the parameters of the system, like the interaction strength, is changed in a sudden manner. This allows to discuss the relaxation behavior on the time scales accessible by the methods and to search for (quasi-)stationary or steady states in the course of the time evolution.

 

Dual-fermion approach to superconductivity in strongly correlated systems: Application to Hubbard and Kondo lattice models

 

Junya Otsuki (Tohoku University)

 

Magnetism and superconductivity appear nearby in typical phase diagrams of d- and f-electron compounds. Addressing such systems requires a unified treatment of magnetism and superconductivity. So far, unconventional superconductivities have been treated by variants

of perturbation theory such as the fluctuation exchange approximation (FLEX). On the other hand, strong local correlations, which are responsible for magnetism, can be described by the dynamical

mean-field theory (DMFT). Therefore, it is highly desirable to construct a FLEX-like approximation on the top of the DMFT.

 

The dual-fermion approach is a framework which enables us to perform a diagrammatic expansion around the DMFT [1]. With this framework, we evaluate FLEX-type diagrams to take account of antiferromagnetic fluctuations. We thus achieve combined descriptions of strong correlations by the DMFT and unconventional superconductivity in the FLEX-like approximation.

 

In my talk, I will present our recent numerical results for superconductivity in the two-dimensional Hubbard model [2] and the Kondo lattice model [3].

 

[1] A. N. Rubtsov et al., Phys. Rev. B 79, 045133 (2009).

[2] J. Otsuki, H. Hafermann, A. I. Lichtenstein, Phys. Rev. B 90, 235132 (2014).

[3] J. Otsuki, arXiv:1504.05637.

 

Worldline Quantum Monte Carlo Analysis of Critical Phenomena in Spin-Boson Systems
 

Hidemaro Suwa (Department of Physics, The University of Tokyo)

 

We have developed efficient worldline Monte Carlo methods for systems with spin-boson interaction and investigated the critical phenomena of spin-phonon systems. Our model describes well the finite-temperature spin-Peierls transition reported experimentally and also the quantum phase transition that the recent developed quantum simulators can realize in trapped-ion systems. In our novel Monte Carlo method, transition probabilities are optimized beyond the detailed balance[1], an extended worm algorithm enables us to calculate off-diagonal correlation[2], and excitation energies are precisely estimated by the generalized moment method[3]. We can calculate the excitation velocity[4] and also the dynamical exponent at a quantum critical point by means of a stochastic approximation[5]. From the combination of these new methods, it is established, for the S=1/2 spin-Peierls chain, that the quantum phonon with a finite frequency is essential to the critical theory governed by the antiadiabatic limit, i.e., the k=1 SU(2) Wess-Zumino-Witten model. Our method is applicable to systems with a higher dimension, phonon frustration, and impurity.

[1] H. Suwa and S. Todo, Phys. Rev. Lett. 105, 120603 (2010)
[2] H. Suwa, Springer Theses (2014)
[3] H. Suwa and S. Todo, arXiv:1402.0847
[4] A. Sen, H. Suwa, and A. W. Sandvik, arXiv:1505.02535
[5] S. Yasuda, H. Suwa, and S. Todo, arXiv:1506.04837

We study the ultrafast dynamic process in photoexcited systems and find that the Franck-Condon or Landau-
Zener tunneling between the photoexcited state and the ground state is abruptly blocked with increasing the state
coupling from nonadiabatic to adiabatic limits. The blockage of the tunneling inhibits the photoexcited state
from decaying into the thermalized state and results in an emergence of a metastable state, which represents an
entanglement of electronic states with different electron-phonon coupling strengths. Applying this model to the
investigation of photoexcited half-doped manganites, we show that the quantum critical transition is responsible
for more than a three-order-of-magnitude difference in the ground-state recovery times following photoirradiation.
This model also explains some elusive experimental results, such as photoinduced rearrangement of orbital order
by the structural rather than electronic process and the structural bottleneck of a one-quarter period of the
Jahn-Teller mode. We demonstrate that in the spin-boson model there exist unexplored regions not covered in
the conventional phase diagram.

Point contact spectroscopy has been extensively used in heavy fermion experiments, but a perfect microscopic understanding is still rare. In this talk I propose some key points to understand point contact spectroscopy of heavy fermion matetial, which are : Fano interference, nonlocal path interference and the strength of coupling between the probe and sample. And I propose that the latter two points also play a key role in distinguishing point contact spectroscopy from scanning tunneling microscope measurement. The toy model used here is a normal metal-heavy fermion sample junction and the conductance is calcultated in non-equilibrium Green function formalism. 

Employing large-scale quantum Monte Carlo simulations, we reveal the full phase diagram of the extended Hubbard model of hard-core bosons on the pyrochlore lattice with partial fillings. When the inter-site repulsion is dominant, the system is in a cluster Mott insulator phase with integer number of bosons localized inside the tetrahedral units of the pyrochlore lattice. We show that the full phase diagram contains three cluster Mott insulator phases with 1/4, 1/2 and 3/4 boson fillings, respectively. We further demonstrate that all three cluster Mott insulators are Coulomb liquid phases and its low-energy property is described by the emergent compact U(1) quantum electrodynamics (QED). Besides measuring the specific heat and entropy of the cluster Mott insulators, we investigate the correlation function of the emergent electric field and verify it is consistent with the compact U(1) QED description. Our results shed lights on magnetic properties of various pyrochlore systems as well as the charge physics of the cluster magnets.

We study the mechanism of decay of a topological (winding-number) excitation due to finite-size effects in a
two-dimensional valence-bond solid state, realized in an S = 1/2 spin model (J − Q model) with six-spin interac-
tions and studied using projector Monte Carlo simulations in the valence bond basis. A topological excitation with
winding number |W | > 0 contains domain walls, which are unstable due to the emergence of long valence bonds in
the wave function, unlike in effective descriptions with the quantum dimer model (which by construction includes
only short bonds). We find that the lifetime of the winding number in imaginary time, which is directly accessible
in the simulations, diverges as a power of the system length L. The energy can be computed within this time (i.e., it
converges toward a “quasieigenvalue” before the winding number decays) and agrees for large L with the domain-
wall energy computed in an open lattice with boundary modifications enforcing a domain wall. Constructing a
simplified two-state model which can be solved in real and imaginary time, and using the imaginary-time behavior
from the simulations as input, we find that the real-time decay rate out of the initial winding sector is exponentially
small in L. Thus, the winding number rapidly becomes a well-defined conserved quantum number for large
systems, supporting the conclusions reached by computing the energy quasieigenvalues. Including Heisenberg
exchange interactions which bring the system to a quantum-critical point separating the valence-bond solid from
an antiferromagnetic ground state (the putative “deconfined” quantum-critical point), we can also converge the
domain-wall energy here and find that it decays as a power law of the system size. Thus, the winding number is
an emergent quantum number also at the critical point, with all winding number sectors becoming degenerate in
the thermodynamic limit. This supports the description of the critical point in terms of a U(1) gauge-field theory.

We investigate the quantum phase transition in an $S = 1/2$ dimerized Heisenberg antiferromagnet in three spatial dimensions. By performing large-scale quantum Monte Carlo simulations and detailed finite-size scaling analyses, we obtain high-precision results for the quantum critical properties at the transition from the magnetically disordered dimer-singlet phase to the antiferromagnetically ordered N\'eel phase. This transition breaks O($N$) symmetry with $N = 3$ in $d = 3 + 1$ dimensions, which is the upper critical dimension in this case. We extract the multiplicative logarithmic corrections to the leading mean-field critical properties in this universality class, testing their precise forms (including exponents) for both the zero-temperature staggered magnetization $m_s$ and the N\'eel temperature $T_N$. A scaling ansatz together with our numerical data indicate a complete absence of logarithmic corrections in $T_N$ expressed as a function of $m_s$. This linear relation implies a full decoupling of quantum and thermal fluctuations even arbitrarily close to the quantum critical point. We also demonstrate agreement with the predicted $N$-independent leading and subleading logarithmic corrections in the size-dependence of the staggered  magnetic susceptibility.  The logarithmic scaling forms have not previously been verified by unbiased numerical methods and we discuss their relevance to experimental studies of dimerized quantum antiferromagnets such as TlCuCl$_3$.

In a Kondo problem in the classical sense (spin 1/2 impurity), either a spin singlet ground state is formed (impurity is screened) or the impurity decouples from the host fermions. Previous studies about Kondo impurity in Luttinger liquid have tell us that the impurity is always screened for the repulsive interaction case. Our study from the strongly repulsive interaction side tell a very different story. We prove that in the case with infinite repulsive interaction  between the itinerant electrons a ferromagnetic ground state is formed. We then apply bosonization method and numerical exact diagonalization method to study the region with finite but strongly repulsive interaction in more details.

Distinguishing nontrivial symmetry protected topological (SPT) phase from trivial insulator in the presence of electron interaction is an urgent question to the study of topological insulators, due to the fact that most of the topological indices defined for free electron systems are very likely unsuitable for interacting cases. In this work, we demonstrate that the strange correlator is an effective diagnosis to detect SPT states in interacting systems. Employing large-scale quantum Monte Carlo (QMC) simulations, we investigate the interaction driven quantum phase transition in the Kane-Mele-Hubbard model. The transition from quantum spin Hall insulator at weak interaction to antiferromagnetic Mott insulator at strong interaction can be readily detected by the momentum space behavior of the strange correlator in single particle, spin, and pairing channels. The interaction effects on the symmetry protected edge states in various channels, i.e., the Luttinger liquid behavior, are well captured in the QMC measurements of strange correlators. Moreover, we demonstrate that the strange correlator is technically much easier to implement in QMC and much robuster in performance than other proposed numerical diagnoses for interacting SPT states, as only static correlations are needed. The attempt in this work paves the way for using the strange correlator to study interaction driven topological phase transitions in fermionic as well as bosonic systems.

The process of converting heat into electricity is an important one for practical applications.  We report some of our theoretical work in this effort.   With the nonequilibrium Green’s function (NEGF) method, we demonstrate that disorder enhances thermoelectric figure of merit (ZT) in armchair graphane nanoribbons.  Most recently, we look at the effect of electron-phonon interaction in quantum dots and find that small coupling already lead drastic reduction of ZT, using quantum master approach.  Finally, report the new finding of enhancing thermoelectric efficiency by time-dependent control.

 

Refs:

X. Ni, G. Liang, J.-S. Wang, and B. Li, “Disorder enhances thermoelectric figure of merit in armchair graphane nanoribbons,” Appl. Phys. Lett. 95, 192114 (2009).

H. Zhou, J. Thingna, J.-S. Wang, and B. Li, “Thermoelectric transport through a quantum nanoelectromechanical system and its backaction,” Phys. Rev. B 91, 045410 (2015).

H. Zhou, J. Thingna, P. Hänggi, J.-S. Wang, and B. Li, “Boosting thermoelectric efficiency using time-dependent control,” arXiv:1505.06132

The many-anyons wavefunction is constructed via the superposition of all the permutations on the direct product of single anyon states. Its interchange and moreover permutation properties are examined. The phase of permutation is not a representation but the word metric of the permutation group. Amazingly the interchange phase yields a finite capacity q of one quantum state interpolating between fermion and boson, as q-exclusion principle. The mutual exchange phase has no explicit effect on statistics. Finite capacity of quantum state is defined as Gentile statistics,  different from the fractional exclusion statistics. Some discussion on the general model may  also  be given. 

We study systematically the effect of long-range dipolar interactions on the ground-state phase diagram of one-dimensional t − J model by using density matrix renormalization group. While the basic phases described by the Luttinger parameter K_ρ, namely, the repulsive Luttinger liquid (metallic phase, K_ρ< 1), attractive Luttinger liquid (superconducting phase, K_ρ> 1), and the phase separation (K_ρ→ ∞) are similar to those of the conventional t − J model, the presence of the long-range dipolar interactions leads to significant differences. (i) At high density regime, the phase boundaries of these three phases are pushed to even large-J region; At low density regime, (ii) these phase boundaries shift toward small-J region and most importantly (iii) the spin-gap region spreads across the boundary of K_ρ= 1, suggesting an exotic metallic phase with spin gap, which is absent in the conventional t−J model. The result indicates that the long-range dipolar interactions have a significant influence on the ground-state properties of the one-dimensional t − J model. Its implication on the pseudogap phenomenon of the hole-doped cuprates is briefly discussed.

Quantum entanglement has become a standard method in the study of the topological aspects of symmetry protected topological phases. In this talk, I would like to introduce an idea to reveal emergent critical behavior in the entanglement Hamiltonian of an AKLT state with extensive bipartition. Effect of disorder, set up by the disjoint partitions with random segment lengths, is the main focus of the talk. I will demonstrate that the infinite-randomness fixed point (IRFP) physics associated with strong-disorder renormalization group (SDRG) approach can emerge natually. An effective central charge c' = ln 2 can be extracted in the nested entanglement entropy calculation, as predicted in the SDRG context.

A recent experiment on Co-doped LiFeAs shows superconductivity in an insulating valence band, evolved from a metallic band upon doping. Here we examine this phenomenon by studying superconductivity in a three-orbital model relevant to the material. We show that Cooper pairing of the insulating hole band requires a finite pairing interaction strength. For strong coupling, the superconductivity in the hole band is robust against the sink of the hole band below the Fermi level. Our theory predicts a substantial upward shift of the chemical potential in the superconducting transition.

 Rotated Heisenberg  model is a new class of quantum spin models to describe quantum magnetisms
 in systems with strong spin-orbit coupling (SOC). It originates from the strong coupling limit of interacting fermions of
 spinor bosons at integer fillings in a lattice in the presence of a SOC.
 In this talk, we focus on Rotated Ferromagnetic Heisenberg model(RFHM). Rotated Antiferromagnetic Heisenberg model (RAFHM) will also be briefly mentioned.
 We introduce Wilson loops to characterize frustrations and gauge equivalent class.
 For a special equivalent class, we identify a new spin-orbital entangled commensurate ground state.
 It supports not only commensurate magnons, but also a new gapped elementary excitation: in-commensurate magnons with two gap minima continuously
 tuned by the SOC strength. When applying a Zeeman field to the RFH,  we find that the condensations of the in-commensurate magnons lead
 to rich and new quantum phases through new universality class of quantum phase transitions.
 These quantum phases include two collinear states at low and high Zeeman field,
 two co-planar canted states at two dual related SOC strengths respectively, also
 a non-coplanar incommensurate Skyrmion ( IC-SkX ) crystal phases surrounded by the 4 phases.
 We also explore new universality classes of quantum phase transitions between( among ) these phases  and finite temperature transitions above all these phases.
 Experimental implementations and  detections of these new phenomena are discussed.
 Possible implications to condensed matter systems with or without SOC are given.

In this talk I will describe our recent work on when and how the entanglement entropy area law is violated in ground states of high-dimensional systems with short-range couplings, and show that free fermion system is an example in which highly excited states obey entanglement entropy volume law. We further show that under appropriate conditions free fermion system exhibits eigenstate thermalization, thus providing an example of eigenstate thermalization hypothesis (ETH). In this case the entanglement entropy is the same as thermal entropy, revealing entanglement as a possible origin of thermal entropy and statistical mechanical description of many-particle systems.

 

The existence of global symmetry enriches the topological feature of fractionalized phases. In particular, the fractional excitations in the system can transform under symmetry in a fractional way, resulting in different Symmetry Enriched Topological (SET) phases. A lot has been understood regarding what 2D SET phases exit and here we try to address the same question in 3D. A new feature of 3D fractionalized phases is the existence of loop excitations and we discuss first how to describe fractionalized symmetry action on loops. Using a dimension reduction procedure, we show that loop excitations exist as the boundary between two 2D SET phases and it is the difference in SET order that characterize the symmetry action. Moreover, similar to the 2D case, we find that some seemingly possible symmetry fractionalization patterns are actually anomalous and cannot be realized in 3D. We propose the flux fusion method to detect such fractionalization patterns.

Looking for superconductors with higher transition temperature requires a guiding principle. In conventional superconductors, electrons pair up into Cooper pairs via the retarded attraction mediated by electron-phonon coupling. Higher-frequency phonon (or smaller atomic mass) leads to higher superconducting transition temperature, known as the isotope effect. Furthermore, superconductivity is the only instability channel of the metallic normal state. In correlated systems, the above simple scenario could be easily violated. The strong local interaction is poorly screened, and this conspires with a featured Fermi surface to promote various competing electronic orders, such as spin-density-wave, charge-density-wave and unconventional superconductivity. On top of the various phases, the effect of electron-phonon coupling is an intriguing issue.  Using the functional renormalization group, here we investigated the interplay between the electron correlation and electron-phonon coupling in a prototype Hubbard-Holstein model on a square lattice. At half-filling, we found spin-density-wave and charge-density-wave phases and the transition between them, while no superconducting phase arises. Upon finite doping, d-wave/s-wave superconductivity emerges in proximity to spin-density-wave/charge-density-wave phases.  Surprisingly, lower-frequency Holstein-phonons are either less destructive, or even beneficial, to the various phases, resulting in a negative isotope effect. We discuss the underlying mechanism behind and the implications of such anomalous effects.

In two foundational papers published in 1952, Lee and Yang established rigorously the connection between the analytic properties of thermodynamic functions and phase transitions. In particular, they proved that the zero points of the partition functions of lattice gases (called Lee-Yang zeros) are all located along the unit circle in the complex plane of fugacity (Lee-Yang theorem), or equivalently, along the imaginary axis of magnetic field for ferromagnetic Ising models. The Lee-Yang zeros, however, have never been observed in experiments since the complex parameters are generally regarded as unphysical. Recently, we discovered that the quantum coherence of a probe spin coupled to a bath is equivalent to the partition function of the bath, with the evolution time corresponding to an imaginary physical parameter. This makes it possible to experimentally study Lee-Yang zeros in particular and thermodynamics for complex parameters in general. Based on this idea, Lee-Yang zeros have been experimentally observed for the first time. We further find that the Yang-Lee edges, i.e., the starting and ending points of the Lee-Yang zeros, lead to a new type of phase transitions, namely, time-domain phase transitions, which manifest themselves in the probe coherence as sudden changes when the bath approaches to the thermodynamic limit. Starting from that, we developed a systematic theory on phase transitions in the complex plane of physical parameters, which can be measured as abrupt changes of the probe coherence evolutions even at temperatures higher than the critical points for conventional phase transitions. We expect a wealth of new, experimentally verifiable physics to be explored in the complex plane of physical parameters.

This work was supported by Hong Kong RGC/GRF, RGC/CRF, and CUHK VC's One-off Discretionary Fund. I acknowledge collaborations with B. B. Wei, X. H. Peng, S. W. Chen, H. C. Po, H. Zhou, J. Cui, and J. Du.

In two foundational papers published in 1952, Lee and Yang established rigorously the connection between the analytic properties of thermodynamic functions and phase transitions. In particular, they proved that the zero points of the partition functions of lattice gases (called Lee-Yang zeros) are all located along the unit circle in the complex plane of fugacity (Lee-Yang theorem), or equivalently, along the imaginary axis of magnetic field for ferromagnetic Ising models. The Lee-Yang zeros, however, have never been observed in experiments since the complex parameters are generally regarded as unphysical. Recently, we discovered that the quantum coherence of a probe spin coupled to a bath is equivalent to the partition function of the bath, with the evolution time corresponding to an imaginary physical parameter. This makes it possible to experimentally study Lee-Yang zeros in particular and thermodynamics for complex parameters in general. Based on this idea, Lee-Yang zeros have been experimentally observed for the first time. We further find that the Yang-Lee edges, i.e., the starting and ending points of the Lee-Yang zeros, lead to a new type of phase transitions, namely, time-domain phase transitions, which manifest themselves in the probe coherence as sudden changes when the bath approaches to the thermodynamic limit. Starting from that, we developed a systematic theory on phase transitions in the complex plane of physical parameters, which can be measured as abrupt changes of the probe coherence evolutions even at temperatures higher than the critical points for conventional phase transitions. We expect a wealth of new, experimentally verifiable physics to be explored in the complex plane of physical parameters.

This work was supported by Hong Kong RGC/GRF, RGC/CRF, and CUHK VC's One-off Discretionary Fund. I acknowledge collaborations with B. B. Wei, X. H. Peng, S. W. Chen, H. C. Po, H. Zhou, J. Cui, and J. Du.

We propose a new type of two-dimensional (2D) allotropes of group-V elements in octagonal tiling (OT) structures. First-principles calculations indicate that these allotropes are both dynamically and thermally stable. The electronic bandstructures of these 2D allotropes are calculated. At a thickness of one bilayer, the allotropes are insulators with energy gaps ranging from 0.3 to 2.0 eV. In particular, OT-Bi is a 2D topological insulator (TI) with an energy gap of 0.3 eV, which is the largest among the reported elemental 2D TIs, making it a good candidate of 2D TI for room temperature applications. The band gap of OT-Bi can be increased further by applying compressive strain or chemical pressure.

Based on dynamical cluster approximation (DCA) quantum Monte Carlo simulations, we study the interaction-driven Mott metal-insulator transition (MIT) in the half-filled Hubbard model on the anisotropic two-dimensional triangular lattice, where the degree of frustration is varied between the unfrustrated case and the fully frustrated, isotropic triangular lattice. Upon increasing the DCA cluster size, we analyze the evolution of the MIT phase boundary as a function of frustration in the phase diagram spanned by the interaction strength and temperature, and provide a quantitative description of the MIT phase boundary in the triangular lattice Hubbard model. Qualitative differences in the phase boundary between the unfrustrated and fully frustrated cases are exhibited. In particular, a change in the sign of the phase boundary slope is observed, which via an impurity cluster eigenstate analysis, may be related to a change in the nature of the insulating state. We discuss our findings within the scenario that the triangular lattice electron system might exhibit a quantum critical Mott MIT with a possible quantum spin liquid insulating state, such as considered for the organic charge transfer salts κ-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2.

Recent theoretical studies have found a class of fractional topological phases on strongly correlated lattice systems with topological flat bands. These new states are named as fractional quantum anomalous Hall (FQAH) states or fractional Chern insulators (FCIs). In this talk, we will introduce new many-body wave functions for the FCI/FQAH states on the disk geometry, using a mapping relationship between the continuum fractional quantum Hall (FQH) states and the lattice FCI/FQAH states. These systems are loaded with particles which obey the generalized Pauli principle (GPP). For the FQH systems, the GPP can be described by the Jack symmetric polynomials. We obtain the many-body wave functions of the fermionic FCI/FQAH states on the disk geometry with the aid of the GPP and the Jack polynomials. Compared with the exact diagonalization results, the wave-function overlap is higher than 0.97 even when the Hilbert-space dimension is large than 3 million. We also use the GPP and the Jack polynomials to construct the FCI/FQAH edge-excitation states.

We propose a relativistic J_eff=3/2 semimetal with 4d¹ or 5d¹ electrons on a cubic lattice when the strong spin-orbital coupling takes over the Hunds' coupling. A relativistic spinor with spin 3/2  is historically called Rarita-Schwinger spinor. In the massless case,  the  right- and left-handed chiral degrees of freedom of the Rarita-Schwinger spinors are independent. In the lattice model that we propose, the right- and left- handed gapless points in Brillouin zone are separated. We call this linearly dispersed semimetal Rarita-Schwinger-Weyl semimetal, similar to Weyl semimetal for spin 1/2 systems.  There is a network of gapless Fermi arcs in the surface Brillouin zone if n₁+n₂+n₃ is even for the normal vector (n₁,n₂,n₃) of the surface while the surface is insulator if n₁+n₂+n₃ is odd.  

By the photoassociation method, the trimer superfluid phase disappears in the one-dimensional state-dependent optical lattice if the ratio of the three-body interaction W to the trimer tunneling J is kept at W/J=12 [W. Z. Zhang et al., Phys. Rev. A 90, 033622 (2014)]. To search for a trimer superfluid and trimer supersolid, we load the cold atom into two-dimensional lattices, whose coordinate number z and kinetic energy −zJ are respectively larger and lower than those of a one-dimensional lattice. Herein, we study the Bose-Hubbard model, which has an additional trimer tunneling term, a three-body interaction, and a next-nearest repulsion. The on-site trimer and trimer superfluid exist if the on-site two-body repulsion and three-body repulsion are smaller than some thresholds. With atom-tunneling terms, the phase transitions from trimer superfluid phase to both atom superfluid and atom supersolid phases are first order. With W/J=12, in a one-dimensional lattice, the trimer superfluid phase does not exist at all. In contrast, the trimer superfluid phase exists in the lower density regions 0<ρ<2 on the square lattice if J is not very large. The trimer superfluid phase emerges in a wider range 0<ρ<3 in the triangular lattice, or in the cubic lattice (z=6). When the three-body interaction is turned on, a trimer supersolid phase emerges due to the classical degeneracy between the quasitrimer solid and the trimer solid being broken by the quantum fluctuation. The phase transitions from the trimer supersolid phase to the quasitrimer solid are first order and the phase transition from the trimer supersolid phase to trimer solid is continuous. Our results, obtained by mean-field and quantum Monte Carlo methods, will be helpful in realizing the trimer superfluid and supersolid by cold-atom experiments.

正规网络结构与同步稳定性的关系

李浩奇  范岸楠  梁世东

中山大学 物理科学与工程技术学院

 

摘要

   复杂网络的结构及其稳定性是统计物理的研究课题之一，它涉及许多物理和生物过程，有广泛的应用前景。我们应用图论和动力学系统理论研究正规网络的优化结构，最小半径及其与同步和稳定性的关系，分析在不同度、不同半径和对称性下网络的稳定性和同步性，并发现度数大和对称性好的网络同步性比较好。

 

致谢：感谢中山大学基地班人才培养基金和理论物理专项基金的资助

本文结合量子团簇理论和密度矩阵内嵌理论，研究了自旋格点体系的基态性质。 我们首先通过构造杂质波函数，将自旋格点模型对应成杂质问题，实现杂质部分约化密度矩阵嵌入到平均场近似的环境态中，重建杂质与环境的纠缠。然后通过线性变分方法，迭代优化得到杂质波函数能量最低态，此时杂质部分的约化密度矩阵可表示真实格点系统中相应格点的基态性质，该做法是内嵌理论中杂质问题的一种新的解决方案，可证明该方法与动力学平均场理论中杂质解决方案的自洽理论是等价的。通过改变杂质格点簇和环境格点簇的大小和形状，研究杂质部分基态能量的精度。我们发现，簇密度矩阵内嵌理论的精度与杂质和环境连接区域的大小直接相关。在二维J_1-J_2模型中，杂质与环境都采用2×2的格点簇，长程有序的晶体态（如Neel态）基态能量精度可达99.9%（E_0=-0.6693），量子顺磁态基态能量精度96%，这表明量子顺磁态的长程关联随距离衰减较Neel态慢，需更大的毗邻区间以构建更接近真实的嵌入环境。

Ground-state properties of the two-dimensional strongly interacting Fermi atomic gas and the effect of spin-orbit coupling Shiwei Zhang College of William and Mary, Williamsburg, Virginia, USA Exact calculations are performed on the two-dimensional strongly interacting, unpolarized, uniform Fermi gas with a zero-range attractive interaction. We briefly describe recent advances in auxiliary-field quantum Monte Carlo techniques, which enable calculations on large enough lattices to reliably compute ground-state properties in the thermodynamic limit. An equation of state is obtained, with a parametrization provided, which can serve as a benchmark and allow accurate comparisons with experiments. The pressure, contact parameter, and condensate fraction are determined systematically vs. the interaction strength. The momentum distribution, pairing correlation, and the structure of the condensate wave function are computed. A Rashba spin-orbit coupling is then included to examine its interplay with superfluidity and the resulting pairing and spin structures and correlations.

多铁材料的微结构和物理性能研究

杨槐馨¹，李俊¹，李建奇^1,2*

1. 中科院物理研究所 先进材料与结构分析实验室 北京100190
2. 量子物质科学协同创新中心 北京100084

      多铁材料和相关物理问题的研究是凝聚态物理研究的一个热点，多铁材料同时具备两种或两种以上铁性（如铁磁性，铁电性，铁弹性等），并可以通过不同自由度之间的耦合产生一系列新效应和新现象。多铁材料的微结构研究涉及到多个相关自由度的有序态问题，特别是局域原子结构序和畴结构的特征分析，以及耦合过程原子结构和畴结构的动态原位观察。现代电子显微镜具有高空间结构分辨率，结合高低温和原位加载技术，已经成为研究多铁材料结构和分析耦合机制的有效手段。

      在前期工作中，我们主要关注了电子铁电材料中的电荷序的演变规律及其局域结构极化。如电子型铁电体LuFe₂O₄的结构相变和电荷序，及其三维电荷序基态结构特性¹； Fe₂OBO₃中的铁电畴及磁电耦合效应²。虽然电子铁电性的研究是大家很关注的问题，在部分体系中，本征铁电性的机制还存在争议，但这种材料的铁电性起源于电子关联效应，涉及到很多重要的物理问题。

       在近期工作中，我们系统研究了六方结构RMnO₃ (R=Ho-Lu, Y, and Sc)几何阻挫多铁体系，其中，拓扑铁电畴结构和界面物理特性是近期多铁材料研究的热点。例如，在YMnO₃中，铁电畴的构型为涡旋态，类似“苜蓿叶”花样，相邻畴间极化相反, 而且畴界表现出和畴区不同的输运和磁性质。在实验研究过程中，我们发展了SEM二次电子成像技术，实现了该体系中高分辨率、大视场铁电畴直接观察³；通过合成途径选择及元素替代对该体系磁性和铁电畴结构进行了调控，并结合利用球差矫正透射电镜对该体系的局域极化行为进行了研究^4-5。

 李建奇：ljq@iphy.ac.cn

Reference:

1.  Y. Zhang, H. X. Yang, C. Ma, H. F. Tian, and J. Q. Li*, Phys. Rev. Lett. 98, 247602 (2007).
2. H.X. Yang, H.F. Tian, Y.J. Song, Y.B. Qin, Y.G. Zhao, C. Ma, and J.Q. Li*, Phys. Rev. Lett. 106, 016406 (2011).
3. J. Li, H. X. Yang*, H. F. Tian, C. Ma, S. Zhang, Y. G. Zhao and J. Q. Li*, Applied Physics Letters 100, 152903 (2012).
4. J. Li, H. X. Yang, H. F. Tian, S.-W. Cheong, C. Ma, S. Zhang, Y. G. Zhao, and J. Q. Li, Phys. Rev. B 87, 094106 (2013).
5. L. Wang, F.K. Chiang,  J. Li, C .Ma, H. X. Yang*, J.Q. Li*, Chin. Sci. Bull.，59,  5194  (2014).

We explore an instantaneous decoherence correction (IDC) approach [1] for the decoherence and energy relaxation in the quantum-classical dynamics of charge transport in organic semiconducting crystals. [2] These effects, originating from environmental fluctuations, are essential ingredients of the carrier dynamics. The IDC is carried out by measurement-like operations in the adiabatic representation. [3] While decoherence is inherent in the IDC, energy relaxation is taken into account by considering the detailed balance through the introduction of energy-dependent reweighing factors, which could be either Boltzmann (IDC-BM) or Miller-Abrahams (IDC-MA) type. For a non-diagonal electron-phonon coupling model, it is shown that the decoherence tends to enhance diffusion while energy relaxation weakens this enhancement. As expected, both the IDC-BM and IDC-MA achieve a near-equilibrium distribution at finite temperatures in the diffusion process, while the Ehrenfest dynamics renders system tending to infinite temperature limit. [4] The resulting energy relaxation times with the two kinds of factors lie in different regimes and exhibit different dependence on temperature, decoherence time and electron-phonon coupling strength, due to different dominant relaxation process.

[1]  Y. Yao, W. Si, X. Y. Hou, and C. Q. Wu, J. Chem. Phys. 136, 234106 (2012).

[2]  A. Troisi and G. Orlandi, Phys. Rev. Lett. 96, 086601 (2006).

[3]  W. Si and C. Q. Wu, Q. arXiv 1505.02234 (2015).

[4]  S. Ciuchi, S. Fratini and D. Mayou Phys. Rev. B 83, 081202 (2011).

量子模拟是量子信息领域一个非常重要而富有活力的研究方向。它可以在人工平台上构建虚拟的量子材料或物理模型，
模拟物理模型在极端物理条件下的性质。目前国际上主要在冷原子、离子阱、集成光子学等系统进行上述的研究。
 

我们从理论上构建了一个新型的量子模拟研究平台，可以仿真格点上的玻色子系统在各种人工规范场下的物理。其基本
的思想是设计一个能够容纳非常多模式且每个模式能量相同的简并光腔，在这里，这些简并模式是光学Laguerre-Gaussian
模式中的轨道角动量自由度。通过诱导不同模式之间的隧穿，原则上，在一个简并光腔系统中可以模拟一维耦合的谐振子链。
通过级联若干个简并光腔，则可以模拟二维格点上的玻色子系统。在这个工作中，我们设计了如何在级联简并腔系统中模拟
人工的阿贝尔、非阿贝尔规范场，以及如何探测能带中Bloch波的性质，进而探测能带陈数、边缘模等拓扑不变量；如何模拟
拓扑量子相变。

除了原则上可以模拟丰富的物理内容之外，这个系统在物理实现上与相近的系统相较，存在明显的优势。传统的采用单模光腔
级联的物理系统，由于使用的物理资源过多（比如说，为了模拟10*100的格点系统，物理上需要1000个级联的单模光腔来实现
一个二维阵列；而在这个系统中，只需要10个简并光腔），太多的噪声因素使得物理实现几乎不可能。另外，人们也尝试采用
硅基光子学系统，将器件固化在芯片上，但是，这在增加物理可实现性的同时，降低了系统的可调节性，大大减少了在同一平
台上可模拟的物理模型的数目。而该系统则可以在使用很少物理资源的前提下，模拟丰富的物理。

二十世纪九十年代以来，随着半导体材料外延技术的不断进步，高亮度的红光，橙光，蓝光以及更短波长的发光二级管（LED）取得了长足的进步，可见光波段发光二级管已经覆盖了从红光到紫外波段范围。与传统光源相比，发光二极管具有寿命长，功耗低，污染少，抗震等许多优点。目前，蓝光和红光都已能够达到非常高的量子效率，商业化产品十分成熟。但波长从535nm至570nm波段的黄绿光发光二极管效率缺仍然低下，这个难点也被科学界称作“green gap”现象。这个区域是人眼对可见光最敏感的波段，如果能实现高效率黄绿光LED，可以在高流明效率的白光LED、全彩显示等领域获得重要应用，进一步拓展LED在照明领域的市场。 AlGaInP系LED由于高Al组分材料能带由直接带隙转变为间接带隙，其发光效率随着发光波长从570nm向更短波长变化时急剧减小，因而不适合用来制作黄绿光LED。InGaN的禁带宽度覆盖紫外至红外，因此有望被用于实现高效率黄绿光LED。然而InGaN基LED的发光效率通常随波长增长而快速下降，这主要是由于长波长LED中In组分增加导致的材料质量下降和量子限制斯塔克效应（QCSE）所导致的。 针对上述问题，我们从外延技术手段和LED外延结构设计两个方面开展了研究。首先，采用外延InGaN层之前预通In源的方法，消除了由于In偏析造成的组分沿生长方向不均匀的问题，并且观察到InGaN基LED发光效率随波长增大反而升高的现象，这可能对于实现InGaN基长波长LED有一定帮助。其次，我们引入了一种混合多量子阱结构，由4个蓝光量子阱和1个黄绿光量子阱构成，获得了高效黄绿光LED，在20mA注入电流下，其主波长、光功率、流明效率、正向电压分别为560nm、2.14mW、19.58lm/W、3.39V。

Motivated by recent efforts in creating flat bands in ultracold atomic systems, we investigate how to probe a flat
band in an optically trapped spin-orbital-coupled Bose-Einstein condensate using Bragg spectroscopy. We find
that the excitation spectrum and the dynamic structure factor of the condensate are dramatically altered when the
band structure exhibits various levels of flatness. In particular, when the band exhibits perfect flatness around the
band minima corresponding to a near-infinite effective mass, a quadratic dispersion emerges in the low-energy
excitation spectrum; in sharp contrast, for the opposite case when an ordinary band is present, the familiar linear
dispersion arises. Such linear-to-quadratic crossover in the low-energy spectrum presents a striking manifestation
of the transition of an ordinary band into a flat band, thereby allowing a direct probe of the flat band by using
Bragg spectroscopy.

自旋轨道耦合在凝聚态物理中成为一个热点问题。随着冷原子物理的发展，

实验上已经实现人工诱导的1D自旋轨道耦合。理论上，已经提出很多

实验方案来实现2D 自旋轨道耦合，但是实验实现仍然是一个挑战。

这里，我们采用双层方案，发现可以实现2D的自旋轨道耦合，

并且带来一些新的物理。

另外，我们还系统研究了2D光晶格中自旋轨道耦合系统，

发现自旋轨道耦合能带来丰富的现象。

Topological states of matter have attracted considerable attention in condensed matter research in the last decade. In the context of ultracold atoms, the search for topological states has triggered rapid experimental progress in optical lattices. As motivated by the fascinating controllability of interactions in atomic systems, e.g., by using Feshbach resonance or Rydberg dressing, we studied interaction induced topological states, which have fundamental difference from the synthetic gauge field approach. In this talk, I will discuss two such examples. One is a spontaneous quantum Hall state emergent from interaction induced topological spin textures in an atomic Bose-Fermi mixture confined in a triangular lattice [1]. The other is a three-dimensional topological density wave state supporting emergent Weyl fermions, in a Rydberg dressed atomic system where the “long-range” interaction plays an important role [2].  

[1] Zhi-Fang Xu, Xiaopeng Li, Peter Zoller, W. Vincent Liu, Spontaneous quantum Hall effect in an atomic spinor Bose-Fermi mixture, PRL 114, 125303 (2015) 
[2] Xiaopeng Li, S. Das Sarma, Exotic Topological Density Waves in Cold Atomic Rydberg Fermions, Nat Comms 6:7137 (2015)

We present phase diagrams of the one-dimensional (1D) SU(2) $\otimes$ XXZ spin-orbital models with negative exchange parameter and analyze entanglement in various ground and excited states. In the SU(2) $\otimes$ $Z_2$ model with Ising orbital interactions we identify a novel phase with spin-singlet dimer correlations and large spin-orbital entanglement entropy determined for an exactly solved ground state, while all the other phases are disentangled. Orbital quantum fluctuations away from the Ising limit do not remove this phase but are responsible for a continuous transition to the phase with coexisting antiferromagnetic and alternating orbital order. The crossover regime between the two above phases is described by two coexisting order parameters. The quantum phases with spin-singlet (orbital-singlet) dimer correlations survive in the isotropic 1D SU(2) $\otimes$ SU(2) model. Considering the disentangled ferro-ferro phase we find one or two spin-orbital exciton bound states characterized by logarithmic scaling.

The generalized rotating-wave approximation (GRWA) is presented for the
two-qubit quantum Rabi model. The analytical expressions in the
zeroth-order approximation recover the previous adiabatic ones.
The counter-rotating-wave terms can be eliminated by performing the first-order
corrections. An effective solvable Hamiltonian with the same form as the
ordinary RWA one is then obtained, giving very accurate eigenvalues and
eigenstates. Energy levels in the present GRWA are in accordance with the
numerical exact diagonalization ones in a wide range of coupling strengths.
The population dynamics in the GRWA are in quantitative agreement
with the numerical results and exhibit the absence of collapses clearly,
revealing the effects of the counter-rotating wave.

In this work, we propose realistic schemes to realize topologically nontrivial Floquet states by shaking optical lattices, using both the one-dimensional lattice and two-dimensional honeycomb lattice as examples. The topological phase in the two-dimensional model exhibits quantum anomalous Hall effect. The transition between topological trivial and nontrivial states can be easily controlled by both shaking frequency and shaking amplitude. We further investigate the interplay between the topology and the correlation in a Haldane-Hubbard model which can be easily realized by shaking lattice method. In the Haldane-Hubbard model, we shows that the develop of the magnetic order will drive the system from the topologcial non-trivial phase to the topological trivial phase, either by a second order phase transition with gap closing and reopening or by a fisrt order transition avoiding the gap closing. The fluctation effect near the critical point is further considered.

 

The integrated spintronic functionalities of phosphorus-doped zigzag graphene nanoribbons and a molecular junction between graphene nanoribbon electrodes

Lin Zhu*（朱琳）, F. Zou（邹飞）,  Kai-LunYao（姚凯伦）

¹School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China

 

Abstract:

On the basis of the density functional theory combined with the Keldysh nonequilibrium Green's function method, we investigate the spin-dependent transport properties of single-edge phosphorus-doped ZGNR systems with different widths. The results show a perfect spin filtering effect reaching 100% at a wide bias range in both P and AP spin configurations for all systems, especially for 6-ZGNR-P system. Instructively, for the AP configuration, the ↓-spin current of the 4-ZGNR-P system appears a negative differential effect. By analyzing transmission spectrum and spin-resolved band structures of the electrode, we elucidate the mechanism for these peculiar properties. Our findings provide a new way to produce multifunctional spin spintronic devices based on phosphorus-doped zigzag graphene nanoribbons.

Meanwhile, we have studied the spin-polarized transport properties of a high-spin state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and spin rectifying effect, associated with giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost not affected by the electrode temperature. The microscopic mechanism of these functionalities are discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of individual spin-crossover molecule.

【参考文献】/ References:

[1] L. Zhu*, K. L. Yao, Z. L. Liu, “Magnetic and electronic switching properties of photochromic diarylethene with two nitronyl nitroxides”, Appl. Phys. Lett. 97, 202101 (2010).

[2] L. Zhu*, K. L. Yao, Z. L. Liu, “Molecular spin valve and spin filter composed of single-molecule magnets”, Appl. Phys. Lett. 96, 082115 (2010).

[3] L. Zhu*, K. L. Yao, Z. L. Liu, “Ab initio studies of ferromagnetic properties of a nitronyl nitroxide radical and a nucleoside with π-conjugated nitroxide spin label”, Appl. Phys. Lett. 90, 182502 (2007).

[4] L. Zhu*, K. L. Yao, Z. L. Liu, “Spin-polarized transport and the electronic structure of the metallic antiferromagnet Fe(thiazole)₂Cl₂”,  J. Chem. Phys. 131, 204702 (2009).

[5] L. Zhu*, Shiv N Khanna, Quantum Spin Transport through Magnetic Superatom dimer (Cs₈V-Cs₈V), J. Chem. Phys. 137, 164311 (2012).

[6] L. Zhu*, Shiv N Khanna, Unusually large spin polarization and magnetoresistance in a FeMg₈-FeMg₈ superatomic dimer, J. Chem. Phys. 139, 064306 (2013).

[7] L. Zhu*, K. L. Yao, Z. L. Liu, “Spin transport and magnetic properties of a copper(II) coordination organometallic molecule”, J. Magn. Magn. Mater. 344, 14 (2013)

[8] L. Zhu*, K. L. Yao, Z. L. Liu, “Biradical and Triradical organic magnetic molecules as Spin filters and rectifiers”, Chem. Phys. 397, 1 (2012).

 

【通讯作者】/ Corresponding Author:

朱琳 副教授

华中科技大学物理学院

湖北省武汉市洪山区珞瑜路1037号，430074

linzh@hust.edu.cn（电子邮件）

We study the phenomenon that the quantum correlation of some bipartite quantum states can be increased by local operations. We prove the condition for a given quantum channel to create quantum correlation from a classically correlated state. Based on the result, the quantum-correlating power of a local quantum channel is defined, and its properties are studied. By using the concept of quantum steering ellipsoids, we make visualized the effect of local quantum channels, and find a class of initial entangled states whose quantum correlation can be increased.

We present direct experimental investigation on quantum dynamics of the Landau-Zener transition in a superconducting qubit. While sweeping the system through an energy level avoided crossing, we measured the density matrix using state tomography. Then we convert the density matix to the population of the time-dependent adiabatic basis. A transition between adiabatic and impulse region is observed. Similar to the dynamics of topological defect production in non-equilibrium second order phase transition, the Landau-Zener dynamics process exhibits a freeze-out phenomenon. The scaling behavior of the topological defects density vs. qunch time confirms the correspondence of non-equilibrium second order phase transition and Landau-Zener transition.

We theoretically study a one-dimensional quasi-periodic Fermi system with topological p-wave
superfluidity. We investigate its phase diagram as functions of the strength of the quasi-disorder and the amplitude of
the p-wave order parameter, through a number of numerical investigations, including a multifractal
analysis. There are four distinct phases separated by three critical lines, i.e., two phases with
all extended wave-functions (I and IV), a topologically trivial phase (II) with all localized wavefunctions
and a critical phase (III) with all multifractal wave-functions. The phase I is related to
the phase IV by duality. It also seems to be related to the phase II by duality. Our proposed phase
diagram may be observable in current cold-atom experiments, in view of simulating non-Abelian
gauge fields and topological insulators/superfluids with ultracold atoms.

Cavity quantum electrodynamics with ferromagnetic magnons

 

Jian-Qiang You（游建强）

Quantum Physics and Quantum Information Division,

Beijing Computational Science Research Center, Beijing 100084, China

*Email: jqyou@csrc.ac.cn

 

 

Abstract

 

Hybridizing collective spin excitations and a cavity with high cooperativity provides a new research subject in the field of cavity quantum electrodynamics and can also have potential applications in quantum information. Here we report an experimental study of cavity quantum electrodynamics with ferromagnetic magnons in a small yttrium-iron-garnet (YIG) sphere at both cryogenic and room temperatures. We observe for the first time a strong coupling of the same cavity mode to both a ferromagnetic-resonance (FMR) mode and a magnetostatic (MS) mode near FMR in the quantum limit. This is achieved at a temperature ~22 mK, where the average microwave photon number in the cavity is less than one. At room temperature, we also observe strong coupling of the cavity mode to the FMR mode in the same YIG sphere and find a slight increase of the damping rate of the FMR mode. These observations reveal the extraordinary robustness of the FMR mode against temperature. However, the MS mode becomes unobservable at room temperature in the measured transmission spectrum of the microwave cavity containing the YIG sphere. Our numerical simulations show that this is due to a drastic increase of the damping rate of the MS mode.

Quantum error correction (QEC) is required for fault-tolerant quantum computation because of the fragile nature of quantum information. To realize a quantum memory protected by QEC is also considered to be one of the urgent goals for quantum computation. To encode quantum information on Schrodinger cat states in a microwave cavity for QEC has attracted a lot of interests. This scheme benefits from the infinite dimensional Hilbert space of a harmonic oscillator for redundant information encoding, one main single error syndrome to track, and long coherence times of the microwave cavity, thus greatly reducing the requirements on hardware. In this talk, I will discuss the recent experimental efforts to realize such a QEC-protected quantum memory [1].

[1]: Tracking photon jumps with repeated quantum non-demolition parity measurements, Nature 511, 444 (2014)

In the physics of ultracold atoms, the two-body scattering
length is a well known parameter. Much less is known about the analogous parameter for three particles, such as three identical bosons. I show that
one can expand the three-body wave function for zero collision energy
and zero orbital angular momentum at large hyperradii rho. At the order
1/rho^4, the three-body scattering hypervolume D appears. D is directly
related to the three-body parameter g_3 in the low-energy effective
field theory. I also show the values of D for some interaction potentials.
As a fundamental parameter, D affects virtually ALL the properties of three or more bosons at low temperatures.

 

The spin-orbit coupling with bosons gives rise to novel properties that are absent in usual bosonic systems. We consider this class of unconventional Bose Einstein condensations and focuse on their topological properties for both the 2D Rashba and 3D  σ · p-type Weyl spin orbit couplings. The single-particle levels show Landau-level-like quantization in the harmonic trap. Interacting condensates develop the half-quantum vortex structure spontaneously which breaks the time-reversal symmetry and exhibits topological spin textures of the skyrmion type. Rotating spin orbit-coupled condensates exhibit rich vortex structures due to the interplay between vorticity and spin texture. In particular, the 3D Weyl coupling generates topological defects in the quaternionic phase space as an SU(2) generalization of the usual U(1) vortices. In the Mott-insulating states in optical lattices with SOC, quantum magnetism is characterized by the Dzyaloshinskii-Moriya-type exchange interactions. The phase digram of spin orbit-coupled condensates subject to in-plane gradient magnetic field is also considered.

Ref:
[1] X. Zhou, J. Zhou and C. Wu , Phys. Rev. A 84 063624 (2011);
[2] Y. Li, X. Zhou and C. Wu, Phys. Rev. B 85 1251 (2012);
[3] Y. Li, X. Zhou and C. Wu, arXiv:1205.2162 (2012).
[4] Zi Cai, X. Zhou, and C. Wu, Phys. Rev. A 85, 061605(R) (2012).
[5] X. Zhou, Yi Li, Zi Cai, C. Wu,  J. Phys. B: At. Mol. Opt. Phys. 46 134001, 2013.
[6]X. Zhou, Z. Zhou, C. Wu, and G. Guo, Phys. Rev. A 91, 033603 (2015).

量子通讯的实现离不开量子存储器。量子存储器是一个能够按照需要存储和读出量子态的系统，而被存储的是非经典的量子态如单光子、纠缠、压缩态等。多模存储能力是衡量量子存储器工作性能的一个重要指标。空间域的多模存储可以极大地减小对存储器存储时间的要求，因此空间多模存储引起了国内外学者的广泛关注，并在近几年取得了许多重要进展。本文将主要回顾近年来量子图像存储的研究进程，重点介绍我们在量子图像存储方面的最新实验进展。

In this talk, I will describe the quasi-particle property when a small number of impurities are immersed into an ultracold Fermi superfluid. The impurity can be dressed by the Bogoliubov quasi-particles of the superfluid and form a polaron picture. Due to the three-component nature of the system, the polaron can become trimer-like with a non-universal energy spectrum. We identify multiple avoided crossings between impurity- and trimer-like solutions in both the attractive and the repulsive polaron spectra. The widths of avoided crossings strongly depend on the strength of the Fermi superfluid and the effective range of the system. We also demonstrate that the mean-field evaluation of the fermion-impurity interaction energy is inadequate even for small fermion-impurity scattering lengths, due to the essential effects of Fermi superfluid and short-range physics in such a system. These results are consistent with the exact solution of atom-dimer scattering length in the deep molecule side of fermions. 

 

References:

W. Yi and X. Cui, "Polarons in Ultracold Fermi Superfluids", arXiv:1503.04966
X. Cui, "Atom-Dimer Scattering and Stability of Bose and Fermi Mixtures", Phys. Rev. A 90, 041603 (R) (2014)

近几十年，利用创新的方法来操纵电子自旋自由度的自旋电子学作为一种新兴技术迅速崛起，并应用在各个科学领域，如磁性随机内存、自旋场发射晶体管、自旋发光二极管器等，它们具有更快的数据处理速度，更低的消耗，更高的集成密度等优势^[59,60]。为实现高效率的自旋电子器件，控制自旋极化电流的产生和操作成为重中之重。然而直到现在，其还是主要依靠光学技术以及对磁场和铁磁材料的应用。 例如：Murayama团队^[61]研究了将自旋从稀释的磁性半导体量子阱中注入到CdSe量子点中，并指出在量子点中达到了40%的自旋极化率。Hickey等^[62]从实验上探索了将自旋从合金Co₂MnGa中注入到处于磁场中的发光二极管中的半导体量子阱，并探测到自旋极化率约为22%。特别是，电偏置电压或门电压被用来进一步提高自旋控制效率和简化实验要求。Souza等^[63]提出了一种一端与正常金属耦合而另一端与铁磁体耦合（NM-QD-FM）的量子点设备。在他们的工作中，自旋极化电流的最大值与等于铁磁电极的极化度。Li等人^[64]研究了在一个量子点一端与正常电极耦合而另一端与铁电极连接的系统的电子自旋极化输运。他们发现在库仑阻塞区域，在强磁场下电流自旋极化可达到100%。

最近，Huber^[15]团队指出，太赫兹辐射可对半导体纳米结构中载流子的轨道和自旋进行相干量子控制从而产生许多新的现象，如拉比振荡，光子回声和载流子相干控制等，这些特性都可以运用在自旋电子学中。作为工业重要基础的太赫兹频域在过去的十年里引起了广泛的科学兴趣^[65,66]。然而，太赫兹仪器，特别是位于1-10THz频域内，没有得到充分发展，产生了所谓的太赫兹间隙。最近，实验上已经在碳纳米管^[13,67,68]和自组装InAs量子点^[16,17,69]通过太赫兹光辅助隧穿得到了太赫兹探测器的实现，并且发现不管是电荷还是轨道量子能级都是10-40meV，即正好对应于太赫兹频域（2.5-10THz）。我们主要探讨了处于磁场和太赫兹辐射下的 InAs量子点的自旋极化率。在对称太赫兹辐射下，自旋电流在不同能级处展现了典型的光辅助隧穿现象——光辅助旁带峰以及在更大的门电压范围内获得100%自旋极化率。此外，我们还研究了外磁场强度的影响：结果表明在低磁场下，由于在自旋向下能级处的光辅助隧穿自旋电流以及自旋电子从两端电极通过量子点能级 自旋向上能级处的顺序隧穿的存在，自旋极化率会表现出振荡特性以及显著的100%自旋极化率平台。最后，我们还发现随着太赫兹辐射强度的增大，自旋极化率也会显示振荡特性。

 

Acoustic particle-trapping by surface structured solid plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

 

Acoustic particle-trapping by surface structured solid platet

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

 

Acoustic particle-trapping by surface structured plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

 

Acoustic particle-trapping by surface structured plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

Rabi模型的分析与计算

张盼  胡洁

首都师范大学物理系

 

Rabi模型是量子光学的基础 ，它描述了一个双能级系统与原子的相互作用。但尽管Rabi模型在物理学中有重要地位，它并没有被精确求解。将Rabi模型在旋波近似下转化为JC模型，然而JC模型只能描述量子光腔的强耦合作用,不能描述超强耦合作用。在这个工作中，我们为了求解Rabi模型，对其哈密顿量进行 的幺正变换，忽略 和 的高阶项，将其投影在 的本征态上，其中 ，对所得哈密顿量进行分析，可以得出能量谱和波函数。在MATLAB中进行数值模拟，将数值模拟的结果与解析结果进行对比，我们发现在低能级区物理图像吻合良好。

 

Analysis and calculation of Rabi model

Pan Zhang  Jie Hu

Capital Normal University

The Rabi model is the basis of quantum optics, which describes the interaction between a two - level system and atom. But although the Rabi model is important in physics, it has not been solved accurately.The Rabi model can be transformed into JC model under the rotating wave approximation.Meanwhile,the JC model only can describe the quantum dynamics of optical-cavity electrodynamics with strong coupling.however,it can not describe solid-state quantum electrodynamics with ultrastrong coupling. In this work, in order to solve this problem ,we let the Rabi Hamiltonian under the unitary transformation with ,and  neglecting and  higher order terms,in the eigenstates of with ,we analyze the obtained Hamiltonian,then we can get the energy spectrum and wave function. We make numerical simulation in MATLAB, and compared with the analytical results, We find that the two physical images in the low level region are in good agreement .

 

Acoustic particle-trapping by surface structured plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

 

Acoustic particle-trapping by surface structured plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

 

Acoustic particle-trapping by surface structured plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

 

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

Acoustic particle-trapping by surface structured plates

Hai-Long He and Ke Deng[1]

Department of Physics, Jishou University, Jishou 416000, Hunan, China

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

 

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

 

 

Department of Physics, Jishou University, Jishou 416000, Hunan, China,

 

Abstract: We investigated the acoustic radiation force acting on a cylindrical (spherical) brass particle near an acoustically soft (stiff) plate patterned with a periodic two-dimensional (three-dimensional) grating. The existence of negative acoustic radiation forces by which the particle can be pulled towards the sound source was confirmed. It was further demonstrated by field distribution analysis that such trapping forces are caused by the gradient vortex velocity field near the plates’ surface, which stems from the collectively resonant excitation of the coupled Scholte surface mods in the thin plates. The revealed acoustical trapping effect has potential use in applications requiring particle manipulation using acoustic waves.

 

Figure 1 (a). Schematic view of an acoustically soft plate patterned with a periodic two-dimensional grating, with a cylindrical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 2D system at the resonance frequency of 0.383(c₀/d).

 

Figure 2 (a). Schematic view of an acoustically stiff plate patterned with a periodic three-dimensional grating, with a spherical brass particle placed nearby. (b) Acoustic radiation force exerted on the particle as a function of frequency. (c) Vortex velocity field distributions of the 3D system at the resonance frequency of 0.505(c₀/d).

 

References

1  H. He, S. Ouyang, Z. He, K. Deng, and H. Zhao, J. Appl. Phys. 117, 164504 (2015).

 

Interest in exploiting the unique properties of metal nanostructures continues to grow nearly unabated, due to its unique size and shape dependent optical, magnetic, and catalytic properties, and potential applications in catalysis,biolabeling, photonics, and information storage etc. Pure phase bismuth nanospheres have been successfully prepared by an thermolysis of bismuth acetate in oleylamine. The size distribution of the bismuth nanospheres was improved by quickly quenching the reaction as revealed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ultraviolet-near-infrared (UV-NIR) absorbance spectrum of the bismuth nanospheres showed camel-like shapes located at 425 nm and 575 nm, which could be ascribed to the effect of the surface plasmon resonance and light scattering. Due to the absorption in the visible range, the prepared bismuth nanospheres showed good photocatalytic properties to the degradation of RhB.

 

量子比特与光场的耦合系统是实现量子计算的一种重要途径，它可以通过光学微腔、超导量子电路等来实现。由于两比特量子门是实现通用量子们的基础，而同时两量子比特可以通过光场来相互作用，从而实现相干态的存储和传输，因此，两量子比特与光场的相互作用系统在量子计算中有着非常重要的作用。由于实验技术的发展，人们已经可以实现量子比特与光场的超强耦合，由此可以加量子门反应的速度，所以我们用适用于比特-光场全耦合区的两比特量子Rabi模型在理论上来研究它。分别利用扩展相干态表象以及Bargmann表象两种方法，我们得到了该模型的解析解。我们发现该模型具有类似于“暗态”的一些特殊本征态，他们在比特-光场全耦合区能量为常数，和“暗态”一样，但不同的是，他们的波函数与耦合强度有关，因此这是一种新的特殊本征态，可能具有与“暗态”类似的应用。同时我们进一步发现，在多比特与光场耦合强度相等的情况下，其他类似的模型也会具有这种类似“暗态”的特殊本征态。例如两比特J-C模型，它具有多种这样特殊的“暗态”。利用两比特Rabi模型的解析解，我们可以来研究系统的性质和时间演化问题，为量子门的构建打下理论基础，同时，特殊“暗态”也存在可能的应用。

Silicon single-photon avalanche diode (SPAD) operating in Geiger mode has become a standard device to detect ultra-weak optical signal in many fields, such as astronomy, biology, lidar, quantum optics, quantum information. In these applications, both high detection efficiency and low timing jitter are required. Currently, there are two primary types of Si SPADs, thick SPAD and thin SPAD, because of its large depletion region thick SPAD can achieve high detection efficiency, at the same time large depletion region also lead to a limited timing performance. Compared to thick SPAD, thin SPAD have a thin depletion region, and its timing jitter performance is great. But thin depletion region can no fully absorb single photon, especially in infrared range, the detection efficiency of thin SPAD is low. Here, we propose a nanostructured SPAD that has the remarkable performance of high detection efficiency over a broad spectral range and low timing jitter at the same time. Nanostructured SPAD have nanocone structure on the both side of single photon avalanche photondiode, and this will great improve the absorption performance of SPAD. Our approach effectively solves the coexistence problem of high efficiency and low jitter for silicon SPADs. 

Monopole condensation phase transition out of quantum spin ice

Gang Chen (陈钢)

chggst@gmail.com

Fudan University

One important goal of condensed matter physics is to identify new phase of matter. Topologically ordered and/or exotic phases of matter are particularly difficult to confirm experimentally because these phases do not have any local order parameter for the experimental detection. Here we propose the idea of using proximate phase transition to indirectly identify such unconventional and featureless phases. We apply this idea to identify quantum spin ice in a realistic physical system. 

Quantum spin ice is a U(1) quantum spin liquid on the pyrochlore lattice and is characterized by emergent U(1) gauge structure and deconfined spinon excitation. Despite the proposal of several candidate f-electron pyrochlore system, the direct observation of the characteristic properties of quantum spin ice is very challenging and is partly because of the very low energy scale of the f-electron local moments. We instead study the proximate magnetic phase transition that is driven by the magnetic monopole condensation in quantum spin ice. Because magnetic monopole is an emergent degree of freedom, such a transition is expected to be unconventional and is characterized by various unusual scaling laws. We use our theory to explain the puzzling experiments in a pyrochlore system.  

Quantum Anomalous Hall Insulator of Composite Fermions

Junren Shi
International Center for Quantum Materials, Peking University

We show that a weak hexagonal periodic potential can transform a two-dimensional electron gas with an even-denominator magnetic filling factor into an quantum anomalous hall insulator of composite fermions, giving rise to the fractionally quantized Hall effect. The system provides a realization of the Haldane honeycomb-net model, albeit in a composite fermion system. Possible experimental setups will also be discussed.  [Y. Zhang and J. Shi, Phys. Rev. Lett. 113, 016801 (2014).]