时间：2023年3月21日（周二）下午 14:00-15:30

Time：14:00-15:30 , Tuesday, March 21st , 2023

主持人: 西湖大学理学院PI 刘仕 博士

Host: Dr. Shi Liu, PI of School of Science, Westlake University

地址：西湖大学云谷校区E10-201阶梯教室

Venue: E10-201, Yungu Campus, Westlake University

何力新

中国科技大学物理学院

Lixin He

School of Physics, University of Science and Technology of China

主讲人/Speaker：

何力新教授1994年毕业于中国科技大学物理系, 并于1997年获中国科技大学硕士学位。 1998 - 2003年在美国Rutgers 物理系师从 David Vanderbilt 教授学习第一性原理的计算方法，并获博士学位。2003.9-2006.1在美国国家再生能源实验室Dr. Alex Zunger 领导的固体理论小组从事半导体量子点的理论研究工作。2006年1月作为国外杰出人才 (百人计划)引进至中国科技大学中科院量子信息重点实验室工作。 2010年获基金委杰出青年科学基金。2012年入选 IOP fellow （UK）。现任科技部量子调控“量子通信网络和量子仿真关键器件的物理实现”(2011-2015) 首席科学家。领导发展了具有自主知识产权的第一性原理计算软件ABACUS。

Professor Lixin He graduated from the Department of Physics at the University of Science and Technology of China in 1994 and obtained a master's degree from the same university in 1997. From 1998 to 2003, he studied first-principles computational methods under the supervision of Professor David Vanderbilt in the Department of Physics at Rutgers University in the United States and obtained his Ph.D. degree. From September 2003 to January 2006, he worked on theoretical research on semiconductor quantum dots in the Solid State Theory Group led by Dr. Alex Zunger at the National Renewable Energy Laboratory in the United States. In January 2006, he joined the Key Laboratory of Quantum Information, CAS at the University of Science and Technology of China. He was awarded the Outstanding Youth Science Fund of the National Natural Science Foundation of China in 2010 and was elected as an IOP Fellow (UK) in 2012. He was the chief scientist of the Quantum Control Program of the Ministry of Science and Technology, focusing on the physical implementation of quantum communication networks and key devices for quantum simulation (2011-2015). He has led the development of the first-principles computational software ABACUS.

讲座摘要/Abstract:

我将介绍我们团队开发的基于第一性原理紧束缚模型的材料计算软件PYATB。PYATB能够通过与基于数值原子轨道基组的第一性原理计算软件ABACUS的接口，直接生成紧束缚模型，避免了生成最大局域化Wannier函数的过程。PYATB能够高效的计算能带及相关的性质，包括材料的拓扑和线性及非线性光学性质。在本报告中，我将介绍两个利用PYATB研究材料性质的例子。在第一个例子中，我们通过能带反折叠方法研究了MnBi₂Te₄表面磁性对其拓扑表面态的影响，发现当最外层Mn原子的磁序减弱时，可以大幅度降低其表面态的能隙，但由于第二层Mn原子磁性的影响，表面整体呈现铁磁性，证实了表面磁性可以与“无”能隙的拓扑表面态共存。在第二个例子中，我们研究了单层SnTe铁电材料中的位移电流效应。我们发现单层SnTe中存在巨大的非线性光位移电流效应，但其线性光学响应极弱，导致了这种材料具有极大的Glass系数。我们分析了这种巨大位移电流的产生机制，发现这是由于在光跃迁矩阵组成的黎曼空间中存在奇点，导致位移矢量发散而引起的。这种激发态空间的奇点可以类比于基态能带的Wyle点。

In this talk, I will introduce the PYATB code developed by our team. PYATB is designed to study the properties of materials based on first-principles tight-binding model. PYATB can directly generate tight-binding models by interfacing with ABACUS, a first-principles calculation software based on numerical atomic orbital basis set, which avoids the process of generating maximally localized Wannier functions. PYATB can efficiently calculate band structures and related properties, including the topological and linear and nonlinear optical properties. In this talk, I will present two examples of using PYATB to study material properties.

In the first example, we studied the effect of surface magnetism on the topological surface states of MnBi2Te4 using the band unfolding method. We found that the surface gap of the material can be significantly reduced when the magnetic order of Mn atoms in outermost layer weakens. However, due to the magnetization of the second-layer Mn atoms, the surface exhibits ferromagnetism as a whole. This confirms that surface magnetism can coexist with the “gapless” topological surface states.

In the second example, we studied the shift current effects in the ferroelectric SnTe monolayer. We found that there is a giant nonlinear shift current effect in SnTe monolayer, but its linear optical response is extremely weak, resulting in an enormous Glass coefficient for this material. We analyzed the mechanism behind this giant shift current effect and found that it is caused by singularities in the Riemann space composed of the optical transition matrix, which lead to divergence of the shift vector. The singularity in the excited state space can be analogous to the Wyle points in the ground-state band structure.

讲座联系人/Contact:

理学院，陈艳艳，邮箱：chenyanyan@westlake.edu.cn

School of Science, Yanyan Chen, Email: chenyanyan@westlake.edu.cn