Infrared spectroscopy of two-dimensional electron systems

Abstract

We have used grazing-angle infrared spectroscopy with polarized radiation to detect the Berreman effect (BE) in the two-dimensional electron systems (2DES) which form spontaneously at two interfaces: one between an amorphous film LaAlO₃ and its SrTiO₃ substrate (LAO/STO), and another at the interface between the topological insulator (TI) Bi₂Se₃ and its sapphire substrate. In both systems we have thus extracted the 2DES parameters at different temperatures. In the quasi-2DES under amorphous LAO, the surface density n_s is higher than under crystalline LAO, while the mobility is nearly the same and the thickness d is 7 nm. In ultrapure Bi₂Se₃ on sapphire, preliminary data provided d < 1 nm and n_s ≈ 10¹³ cm⁻², indicating, by comparison with existing estimates, that the BE indeed detects the topological surface states.