Giant Magneto-optical Kerr Effect and Universal Faraday Effect in Thin-film Topological Insulators

TL;DR

This paper demonstrates that thin-film topological insulators exhibit a universal Faraday rotation tied to fundamental constants and a giant Kerr rotation, due to their unique surface states and cavity effects.

Contribution

It reveals universal magneto-optical effects in topological insulator thin films, linking them to fundamental constants and surface Hall conductivity.

Findings

Faraday rotation is universally $ an^{-1} \, \alpha$ at low frequencies.

Giant Kerr rotation of $\pi/2$ observed.

Effects result from surface states and cavity confinement.

Abstract

Topological insulators can exhibit strong magnetoelectric effects when their time-reversal symmetry is broken. In this Letter we consider the magneto-optical Kerr and Faraday effects of a topological insulator thin film weakly exchange-coupled to a ferromagnet. We find that its Faraday rotation has a universal value at low-frequencies, $\theta_{\mathrm{F}} = \mathrm{tan}^{-1}\,\alpha$ where $\alpha$ is the vacuum fine structure constant, and that it has a giant Kerr rotation $\theta_{\mathrm{K}} = \pi/2$. These properties follow from a delicate interplay between thin-film cavity confinement and the surface Hall conductivity of a topological insulator's helical quasiparticles.