Landau level spectroscopy of surface states in the topological insulator Bi0.91Sb0.09 via magneto-optics

We have performed broadband zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi0.91Sb0.09. The zero-field results allow us to measure the value of the direct band gap between the conducting L-a and valence L-s bands. Under applied field in the Faraday geometry (k parallel to H parallel to C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence omega alpha root H. We discuss the ramification of this observation for the surface and bulk properties of topological insulators.