Experimental signature of parity anomaly in semi-magnetic topological insulator

TL;DR

This paper reports the experimental observation of the parity anomaly in a semi-magnetic topological insulator, demonstrating half-integer quantization of Hall conductance through magneto-optical and transport measurements.

Contribution

It provides the first experimental evidence of the parity anomaly in a condensed matter system by isolating a single Dirac surface state in a semi-magnetic topological insulator.

Findings

Half quantized Faraday/Kerr rotations observed

Half quantized Hall conductance measured at zero magnetic field

Demonstration of parity anomaly in a condensed matter system

Abstract

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.