title: Terahertz spectroscopy on Faraday and Kerr rotations in a quantum anomalous Hall state.
abstract: Electrodynamic responses from three-dimensional topological insulators are characterized by the universal magnetoelectric term constituent of the Lagrangian formalism. The quantized magnetoelectric coupling, which is generally referred to as topological magnetoelectric effect, has been predicted to induce exotic phenomena including the universal low-energy magneto-optical effects. Here we report the experimental indication of the topological magnetoelectric effect, which is exemplified by magneto-optical Faraday and Kerr rotations in the quantum anomalous Hall states of magnetic topological insulator surfaces by terahertz magneto-optics. The universal relation composed of the observed Faraday and Kerr rotation angles but not of any material parameters (for example, dielectric constant and magnetic susceptibility) well exhibits the trajectory towards the fine structure constant in the quantized limit.
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.|Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Ken N
authors.initials: KN
authors.lastname: Okada
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.|Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan.|PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: Youtarou
authors.initials: Y
authors.lastname: Takahashi
authors.affiliation: Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Masataka
authors.initials: M
authors.lastname: Mogi
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.|Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Ryutaro
authors.initials: R
authors.lastname: Yoshimi
authors.affiliation: PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0075, Japan.|Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.
authors.forename: Atsushi
authors.initials: A
authors.lastname: Tsukazaki
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.|PRESTO, Japan Science and Technology Agency (JST), Chiyoda-ku, Tokyo 102-0075, Japan.
authors.forename: Kei S
authors.initials: KS
authors.lastname: Takahashi
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
authors.forename: Naoki
authors.initials: N
authors.lastname: Ogawa
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.|Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Masashi
authors.initials: M
authors.lastname: Kawasaki
authors.affiliation: RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan.|Department of Applied Physics and Quantum Phase Electronics Center (QPEC), University of Tokyo, Tokyo 113-8656, Japan.
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Tokura