Quantized Faraday and Kerr rotation and axion electrodynamics of a 3D topological insulator

TL;DR

This study demonstrates quantized Faraday and Kerr rotations in 3D topological insulator Bi$_2$Se$_3$ films, providing evidence for axion electrodynamics and enabling a direct measurement of the fine structure constant.

Contribution

It reports the first observation of quantized Faraday and Kerr rotations in topological insulators, linking topological invariants to measurable optical effects.

Findings

Quantized Faraday and Kerr rotations observed above 5 T

Evidence for axion electrodynamics in a solid-state system

Direct measurement of the fine structure constant

Abstract

Topological insulators have been proposed to be best characterized as bulk magnetoelectric materials that show response functions quantized in terms of fundamental physical constants. Here we lower the chemical potential of three-dimensional (3D) Bi$_2$Se$_3$ films to $\sim$ 30 meV above the Dirac point, and probe their low-energy electrodynamic response in the presence of magnetic fields with high-precision time-domain terahertz polarimetry. For fields higher than 5 T, we observed quantized Faraday and Kerr rotations, whereas the DC transport is still semi-classical. A non-trivial Berry phase offset to these values gives evidence for axion electrodynamics and the topological magnetoelectric effect. The time structure used in these measurements allows a direct measure of the fine structure constant based on a topological invariant of a solid-state system.