Using nonlocal surface transport to identify the axion insulator

Abstract

The axion is a hypothetical but experimentally undetected particle. Recently, the antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ was 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 $\mathrm{Mn}{\mathrm{Bi}}_{2}{\mathrm{Te}}_{4}$ 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.