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Quantum Anomalous Semimetals

Date: 2022-03-23
Time: 10:00
Venue: M234
Speaker: Prof. Shun-Qing Shen

Department of Physics, The University of Hong Kong


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.


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

2. 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

3. 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

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

Brief CV of Prof. Shun-Qing Shen:

Professor Shun-Qing Shen, 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. He proposed topological Anderson insulator, theory of weak-localization and anti-localization in topological insulators, resonant spin Hall effect and theory of phase separation in colossal magnetoresistive (CMR) materials. He proved existence of antiferromagnetic long-range order and off-diagonal long-range order in itinerant electron systems. He has published a single-authored monograph, Topological Insulators (Springer, 2012), which is the first book on the topic.

Professor Shen received his BS, MS, and PhD in theoretical physics from Fudan University in Shanghai. He was a postdoctorial fellow in China Center of Advanced Science and Technology (CCAST), Beijing, Alexander von Humboldt fellow in Max Planck Institute for Physics of Complex Systems, Dresden, Germany, and JSPS research fellow in Tokyo Institute of Technology, Japan. In December 1997 he joined The University of Hong Kong.

Contact: Liu Yang(杨柳)


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