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

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 α/4π per surface. Here we report on the observation of this so-called topological magnetoelectric effect. We use monochromatic terahertz (THz) spectroscopy of TI structures equipped with a semitransparent gate to selectively address surface states. In high external magnetic fields, we observe a universal Faraday rotation angle equal to the fine structure constant α= e ²/2 hc (in SI units) 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.

Abstract

The electrodynamics of topological insulators has been predicted to show a new magnetoelectric term, but this hasn't been observed. Here, Dziom et al. observe a universal Faraday rotation angle equal to the fine structure constant, evidencing the so-called topological magnetoelectric effect.