报告题目：Half Quantized Hall Effect in Metal

报告人：沈顺清 教授（香港大学）

邀请人：李朝红 教授

时  间：2024年01月19日（周五）16:00-17:00

地  点：深圳大学粤海校区光电所313会议室

报告摘要：

 量子霍尔效应是指在极低温度下和强磁场中，二维电子气系统中发现的一系列奇异的物质量子状态，它的霍尔电导是量子化的。在没有磁场的情况下，二维材料中的类似现象被称为量子反常霍尔效应。到目前为止，所有的量子霍尔效应都发生在绝缘相中，并以整数或有理分数为特征。这些量子霍尔效应发生在费米能级位于朗道能级的能隙或能带隙内时，其特征在于能带结构的 TKNN 数或陈数作为拓扑不变量。纵向电导率为零，霍尔电阻率或电导率是整数量子化的。体与边缘对应表明陈数对应于系统边界周围的局部边缘模式的数量，其承载无耗散手性电流。在这里，我们将介绍金属或半金属中的半量化霍尔效应。霍尔电导是半整数量子化的，纵向电导不为零。而相应的霍尔电阻率并不是量子化的。当费米面在宇称对称性下是不变的，而整个系统的对称性破缺时，就有可能导致霍尔电导半整数量子化。最近的一项实验报告了对磁掺杂拓扑绝缘体中半量子化霍尔电导的观测。我们发现，当费米能级截取布里渊区有限范围内宇称不变的无带隙表面态时，能带结构中存在单个无带隙狄拉克锥并具有半量子化电导。由于在无间隙和金属系统中没有手性边缘状态，我们还发现手性边缘电流由无能隙表面状态携带。电流密度在边缘处达到峰值并按幂律衰减，而通常量子反常霍尔效应中的边界电流是指数衰减的。我们将这个量子物态称为“宇称反常半金属”。这项工作为探索具有分数拓扑电荷的新拓扑物质状态打开了大门。

The quantum Hall effects refer to a series of peculiar quantum states of matter in the two-dimensional electron system in a strong magnetic field at a very low temperature. Similar phenomena in quasi-two-dimensional materials in the absence of a magnetic field are named the quantum anomalous Hall effect. So far, all the quantum Hall effects occur in insulating phases and are characterized by the TKNN number or Chern number for the band structure as a topological invariant. The longitudinal conductivity is zero and either the Hall resistivity or conductivity is quantized. Here we report a half-quantized Hall effect in a metal or semimetal. The Hall conductance is half quantized and the longitudinal conductance is nonzero. Consequently, the Hall resistivity is not quantized. The half quantization occurs when the Fermi surface is invariant under the parity symmetry or time reversal symmetry while the symmetry is broken in the whole system. A recent experiment reports the observation of the half-quantized Hall conductance in a magnetically-doped topological insulator. We discover that a single gapless Dirac cone exists in the band structure and has half-quantized conductance when the Fermi level intercepts the gapless surface states in which the parity symmetry is respected in a finite regime in the Brillouin zone. As there are no localized chiral edge states in the gapless and metallic system, we find that the chiral edge current is carried by the gapless surface states. The current density peaks at the edge and decays in a power law rather than the exponential decay as in the conventional quantum anomalous Hall effect. We term the nontrivial quantum phase as “parity anomalous semimetal”. The work opens the door to exploring novel topological states of matter with fractional topological invariants.

报告人简介:

沈顺清教授是香港大学物理系教授，专攻于凝聚态物理理论。他在拓扑绝缘体、量子输运和新奇量子物态研究方面做出了杰出贡献。他出版了一本名为《拓扑绝缘体》（Springer, 2012）的专著。沈教授在上海复旦大学获得了理学学士，硕士和博士学位。他曾在北京中国高等科学研究中心（CCAST）担任博士后研究员，在德国德累斯顿的马克斯·普朗克复杂系统物理研究所担任洪堡学者，并在日本东京工业大学担任JSPS研究员。他1997年加入香港大学物理系。

Shun-Qing Shen is a professor in the Department of Physics at the University of Hong Kong. He is an expert in the field of condensed matter physics and is distinguished for his research works on topological insulators, quantum transport, and novel quantum states of condensed matter. He published a single-authored monograph, Topological Insulators (Springer, 1st ed., 2012; 2nd ed., 2017), which is the first one on the topic. He received his BS. MS, and Ph.D. in theoretical physics from Fudan University in Shanghai, China. He was a post-doctoral fellow at China Center of Advanced Science and Technology (CCAST), Beijing, an Alexander von Humboldt fellow at Max Planck Institute for Physics of Complex Systems, Dresden, Germany, and a JSPS research fellow at Tokyo Institute of Technology, Japan. He joined The University of Hong Kong in December 1997.

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