Using nonlocal surface transport to identify the axion insulator

The axion is a hypothetical but experimentally undetected particle. Recently, the antiferromagnetic topological insulator MnBi2Te4 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 MnBi2Te4 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.