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

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Science  02 Dec 2016:
Vol. 354, Issue 6316, pp. 1124-1127
DOI: 10.1126/science.aaf5541

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Shining light on a peculiar coupling

One of the long-standing predictions regarding topological insulators is the magnetoelectric effect, a coupling between a material's magnetic and electric properties. Thanks to this coupling, Maxwell's equations inside topological insulators are modified, resulting in so-called axion electrodynamics. Wu et al. used time-domain terahertz (THz) spectroscopy to observe signatures of these unusual electrodynamics in a thin film of Be2Se3. They detected tiny changes to the polarization of THz light after it passed through the thin film, confirming the expected quantization of the magnetoelectric coupling.

Science, this issue p. 1124

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) Bi2Se3 films to ~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 tesla, we observed quantized Faraday and Kerr rotations, whereas the dc transport is still semiclassical. A nontrivial Berry’s 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.

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