Observation of the universal magnetoelectric effect in a 3D topological insulator

TL;DR

This paper reports the first observation of the universal topological magnetoelectric effect in a 3D topological insulator, demonstrating a quantized Faraday rotation linked to fundamental constants using terahertz spectroscopy.

Contribution

It provides experimental evidence of the quantized topological magnetoelectric effect in a 3D topological insulator, confirming theoretical predictions and exploring axion electrodynamics.

Findings

Universal Faraday rotation angle equal to the fine structure constant observed.

Quantized coupling between electric and magnetic fields in topological insulators confirmed.

Insights into axion electrodynamics and potential for fundamental constant metrology.

Abstract

The electrodynamics of topological insulators (TIs) is described by modified Maxwell's equations, which contain additional terms that couple an electric field to a magnetization and a magnetic field to a polarization of the medium, such that the coupling coefficient is quantized in odd multiples of $e^2 / 2 h c $ per surface. Here, we report on the observation of this so-called topological magnetoelectric (TME) effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semi-transparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant $\alpha = e^2 / \hbar c$ when a linearly polarized THz radiation of a certain frequency passes through the two surfaces of a strained HgTe 3D TI. These experiments give insight into axion electrodynamics of TIs and may potentially be used for a metrological definition of the three basic physical constants.