Magneto-Optics in the search for the topological insulating state

We present infrared spectroscopic data for the thermoelectric material Bi-Sb at the insulating composition, taken in magnetic field applied both in the Voight and Faraday geometries, in order to gain insight into the recently discovered topological insulating state. We observe a metallic plasma edge that displays significant blue shift at high temperatures. The form of the plasma edge shows broadening suggestive of sample inhomogeneities. The plasma edge is remarkably sensitive to the applied magnetic field, resulting in a complicated spectral lineshape. We compare these results with magneto-optics of the topological insulator Bi$_{2}$Se$_{3}$.\footnote{A. D. LaForge, \textit{et. al.}, Phys. Rev. B 81, 125120 (2010).}