Infrared Spectroscopy of the Topological Surface States of Bi2Se3 by Use of the Berreman Effect

The Berreman effect (BE) allows one to study the electrodynamics of ultrathin conducting films at the surface of dielectrics by use of grazing-angle infrared spectroscopy and polarized radiation. Here, we first apply the BE to the two-dimensional electron system (2DES) at the interface between a high-purity film of the topological insulator Bi2Se3 and its sapphire substrate. We determine for the 2DES a charge density n(s) = (8 +/- 1) x 10(12) cm(-2), a thickness d = 0.6 +/- 0.2 nm, and a mobility mu(IR) = 290 +/- 30 cm(2)/Vs. Within errors, all of these parameters result in being independent of temperature between 300 and 10 K. These findings consistently indicate that the 2DES is formed by topological surface states, whose infrared response is then directly observed here.