Non-local sidewall response and deviation from exact quantization of the topological magnetoelectric effect in axion-insulator thin films

TL;DR

This paper investigates how non-local sidewall responses cause deviations from perfect quantization in the topological magnetoelectric effect of axion insulator thin films, and how stronger exchange coupling can mitigate these deviations.

Contribution

It provides a unified theoretical framework linking non-local sidewall responses to quantization deviations and shows how enhanced exchange coupling reduces these effects in Bi2Se3 thin films.

Findings

Deviations from QTME quantization are linked to non-local sidewall responses.

Stronger exchange coupling reduces quantization deviations.

Potential disorder impacts QTME more than QAHE.

Abstract

Topological insulator (TI) thin films with surface magnetism are expected to exhibit a quantized anomalous Hall effect (QAHE) when the magnetizations on the top and bottom surfaces are parallel, and a quantized topological magnetoelectric (QTME) response when the magnetizations have opposing orientations (axion insulator phase) and the films are sufficiently thick. We present a unified picture of both effects that associates deviations from exact quantization of the QTME caused by finite thickness with non-locality in the side-wall current response function. Using realistic tight-binding model calculations, we show that in $Bi_2Se_3$ TI thin films deviations from quantization in the axion insulator-phase are reduced in size when the exchange coupling of tight-binding model basis states to the local magnetization near the surface is strengthened. Stronger exchange coupling also reduces the effect of potential disorder, which is unimportant for the QAHE but detrimental for the QTME, which requires that the Fermi energy lie inside the gap at all positions.