## PRESS RELEASE

- Research
- 2022

# Experimental signature of the parity anomaly in a semi-magnetic topological insulator

Authors

M.Mogi, Y. Okamura, M. Kawamura, R. Yoshimi, K. Yasuda, A. Tsukazaki, K. S. Takahashi, T. Morimoto, N. Nagaosa, M. Kawasaki, Y. Takahashi, and Y. Tokura

Abstract

A three-dimensional (3D) topological insulator features a 2D surface state consisting of a single linearly dispersive Dirac cone^{1,2,3}. Under broken time-reversal symmetry, the single Dirac cone is predicted to cause half-integer quantization of Hall conductance, which is a manifestation of the parity anomaly in quantum field theory^{1,2,3,4,5,6,7,8,9}. However, despite various observations of quantization phenomena^{10,11,12,13,14,15}, the half-integer quantization has not been observed because most experiments simultaneously measure a pair of equivalent Dirac cones^{16} on two opposing surfaces. Here we demonstrate the half-integer quantization of Hall conductance in a synthetic heterostructure termed a semi-magnetic topological insulator, where only one surface state is gapped by magnetic doping and the opposite one is non-magnetic and gapless. We observe half-quantized Faraday and Kerr rotations with terahertz magneto-optical spectroscopy and half-quantized Hall conductance in transport at zero magnetic field. Our results suggest a condensed-matter realization of the parity anomaly^{4,5,6,7,8,9} and open a way for studying the physics enabled by a single Dirac fermion.

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Nature Physics：https://www.nature.com/articles/s41567-021-01490-y