Using nonlocal surface transport to identify the axion insulator

TL;DR

This paper proposes a nonlocal surface transport method to identify axion insulators, like MnBi2Te4, by detecting their unique surface current signatures without needing precise quantization measurements.

Contribution

It introduces a robust, qualitative nonlocal transport technique to distinguish axion insulators from normal insulators in realistic conditions.

Findings

Nonlocal surface transport can identify axion insulators.

The method is robust against side surface states and disorder.

Thick electrodes improve measurement feasibility.

Abstract

The axion is a hypothetical but experimentally undetected particle. Recently, the antiferromagnetic topological insulator MnBi$_2$Te$_4$ has been predicted to host the axion insulator, but the experimental evidence remains elusive. Specifically, the axion insulator is believed to carry "half-quantized" chiral currents running antiparallel on its top and bottom surfaces. However, it is challenging to measure precisely the half-quantization. Here, we propose a nonlocal surface transport device, in which the axion insulator can be distinguished from normal insulators without a precise measurement of the half-quantization. More importantly, we show that the nonlocal surface transport, as a qualitative measurement, is robust in realistic situations when the gapless side surfaces and disorder come to play. Moreover, thick electrodes can be used in the device of MnBi$_2$Te$_4$ thick films, enhancing the feasibility of the surface measurements. This proposal will be insightful for the search of the axion insulator and axion in topological matter.