Resonant Faraday and Kerr effects due to in-gap states on the surface of a topological insulator

TL;DR

This paper studies how localized in-gap states caused by impurities influence the magneto-optical Faraday and Kerr effects on the surface of topological insulators, revealing resonant signatures at higher frequencies.

Contribution

It demonstrates that in-gap impurity states resonantly affect the magneto-optical responses, providing a new way to probe impurity effects in topological insulators.

Findings

In-gap states cause resonant features in Faraday and Kerr spectra.

Localized impurity states significantly modify high-frequency magneto-optical effects.

Distinct signatures differentiate impurity states from excitonic contributions.

Abstract

When Dirac electrons on the surface of a topological insulator are gapped, the resulting quantum anomalous Hall effect leads to universal magneto-optical Faraday and Kerr effects in the low frequency limit. However, at higher frequencies different excitations can leave their own fingerprints on the magneto-optics and can therefore be probed. In particular, we investigate the role of localized in-gap states---which inevitably appear in the presence of charged impurities---on these higher frequency magneto-optical effects. We have shown that these states resonantly contribute to the Hall conductivity and are magneto-optically active. These in-gap states lead to peculiar resonant signatures in the frequency dependence of the Faraday and Kerr angles, distinct in character to the contribution of in-gap excitonic states, and can be probed in ellipsometry measurements.