## Quantum Anomalous Semimetals

Speaker: Shun-Qing Shen (Department of Physics, The University of Hong Kong)

Time: March 25, 2022 10:00

Place: 理科楼C302

**报告摘要：**The topological states of matter and topological materials have been attracting extensive interests as one of the frontier topics in condensed matter physics and materials science since the discovery of quantum Hall effect in 1980s. So far all the topological phases such as quantum Hall effect, quantum spin Hall effect and topological insulators and superconductors are characterized by a nonzero integer or Z and Z2 topological invariant. None is a half-integer or fractional. Here we propose a novel type of semimetals which hosts a single cone of Wilson fermions instead of Dirac fermions. The Wilson fermions possess linear dispersion near the energy crossing point, but breaks the chiral or parity symmetry such that an unpaired Dirac cone can be realized on a lattice. They are not prohibited by the Nielsen-Ninomiya theorem and avoid the fermion doubling problem. We find that the system can be classified by the relative homotopy group, and the topological invariant is a half-integer. We term the unexpected and nontrivial quantum phase as “quantum anomalous semimetal”. The topological phase is a synergy of topology of band structure in solid and quantum anomaly in quantum field theory. The work opens the door towards exploring novel states of matter with fractional topological charge.

**报告人简介：**Professor Shen has been a professor of physics at The University of Hong Kong since July 2007. Professor Shen received his BS, MS, and PhD in theoretical physics from Fudan University in Shanghai. He was a postdoctorial fellow (1992 – 1995) in China Center of Advanced Science and Technology (CCAST), Beijing, Alexander von Humboldt fellow (1995 – 1997) in Max Planck Institute for Physics of Complex Systems, Dresden, Germany, and JSPS research fellow (1997) in Tokyo Institute of Technology, Japan. He is an expert in the field of condensed matter physics, is distinguished for his research works on topological insulator, quantum transport, spintronics of semiconductors, quantum magnetism and orbital physics in transition metal oxides, and novel quantum states of condensed matters.

**参考文献：**

Bo Fu, Jin-Yu Zou, Zi-Ang Hu, Huan-Wen Wang, and Shun-Qing Shen, Quantum anomalous semimetals, arXiv: 2203.00933

Huan-Wen Wang, Bo Fu, Jin-Yu Zou, Zi-Ang Hu, and Shun-Qing Shen, Fractional electromagnetic response in three-dimensional chiral anomalous semimetal, arXiv: 2202.01957

Jin-Yu Zou, Bo Fu, Huan-Wen Wang, Zi-Ang Hu, and Shun-Qing Shen, Half-quantized Hall effect and power law distribution of edge current distribution, arXiv: 2202.08493

Huan-Wen Wang, Bo Fu, and Shun-Qing Shen, Helical symmetry breaking and quantum anomaly, Phys. Rev. B 104, L241111 (2021)