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=e²/2"Equation missing"<!-- image only, no MathML or LaTex -->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.

Dielectric spectroscopy up to 370 GHz has been performed on supercooled glycerol for temperatures between 250 and 330 K. The high-frequency dielectric loss, ${\varepsilon }^{\prime \prime }$, has been investigated by coherent-source submillimeter-wave spectroscopy. Special attention is given to the dielectric loss in the crossover regime from the structural relaxation to the far-infrared response. A minimum is observed in ${\varepsilon }^{\prime \prime }\left(\nu \right)$ which is compared to theoretical concepts that predict fast relaxational processes.

Spectral ellipsometry is used to determine the dielectric function of an untwinned crystal of ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3$ in the range 0.5–5.6 eV at temperatures $50\le T\le 300\mathrm{K}$. A pronounced redistribution of spectral weight is found at the Néel temperature $T_N=140\mathrm{K}$. The anisotropy of the spectral weight transfer matches the magnetic ordering pattern. A superexchange model quantitatively describes spectral weight transfer induced by spin correlations. This analysis implies that the lowest-energy transitions around 2 eV are intersite $d$-$d$ transitions, and that ${\mathrm{L}\mathrm{a}\mathrm{M}\mathrm{n}\mathrm{O}}_3$ is a Mott-Hubbard insulator.

We report the observation of a giant Faraday effect, using terahertz (THz) spectroscopy on epitaxial HgTe thin films at room temperature. The effect is caused by the combination of the unique band structure and the very high electron mobility of HgTe. Our observations suggest that HgTe is a high-potential material for applications as optical isolator and modulator in the THz spectral range.