Optical characterization of Bi2Se3 in a magnetic field: Infrared evidence for magnetoelectric coupling in a topological insulator material

We present an infrared magneto-optical study of the highly thermoelectric narrow-gap semiconductor Bi2Se3. Far-infrared and midinfrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal c axis of this layered material and supplemented with UV-visible ellipsometry to obtain the optical conductivity sigma(1)(omega). With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as an increase in the absorption edge due to direct electronic transitions. Magnetic fields H parallel to c dramatically renormalize and asymmetrically broaden the strongest far-IR optical phonon, indicating interaction of the phonon with the continuum free-carrier spectrum and significant magnetoelectric coupling. For the perpendicular field orientation, electronic absorption is enhanced, and the plasma edge is slightly shifted to higher energies. In both cases the direct transition energy is softened in magnetic field.