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

• DOI: 10.1038/s41567-021-01490-y
• Publication Date: 2022-01-27T17:02:41Z
• Authors: M. Mogi; Y. Okamura; M. Kawamura; R. Yoshimi; K. Yasuda; A. Tsukazaki; K. S. Takahashi; T. Morimoto; N. Nagaosa; M. Kawasaki; Y. Takahashi; Y. Tokura
• Keywords: Topological insulator; Parity (physics); Anomaly (physics); Signature (topology)

A three-dimensional topological insulator features a two-dimensional surface state consisting of a single linearly-dispersive Dirac cone. 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. However, despite various observations of quantization phenomena, the half-integer quantization has been elusive because a pair of equivalent Dirac cones on two opposing surfaces are simultaneously measured in ordinary experiments. 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/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 and open a way for studying unconventional physics enabled by a single Dirac fermion.<br/>25 pages, 7 figures<br/>