Side-surface-mediated hybridization in axion insulators

The axion insulator is believed to host half-quantized chiral currents running antiparallelly on its top and bottom surfaces. However, the experimental detection of the half quantization in axion insulators remains elusive. In this paper, we propose a mechanism to explain why the half quantization is hard to be probed by showing that the half-quantized counterpropagating currents in axion insulator thin films are strongly suppressed due to the hybridization mediated by the massless side-surface states. This side-surface-mediated hybridization leads to a different type of finite-size effect, which features a power-law decay with the increasing film thickness, different from the exponential decay in topological insulators. Moreover, we show that the half quantization can be extracted in the axion insulator phase by adopting the nonlocal transport measurement.