题 目：Half Quantized Hall Effect in Metal

报告人：沈顺清 教授，香港大学物理系

时 间：2024年4月1日（周一）10:00

地 点：国家脉冲强磁场科学中心B206

报告摘要：

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：

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.