Landau level spectroscopy of surface states in the topological insulator   Bi$_{0.91}$Sb$_{0.09}$ via magneto-optics
  We have performed broad-band zero-field and magneto-infrared spectroscopy of the three dimensional topological insulator Bi$_{0.91}$Sb$_{0.09}$. 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 (\emph{k} $||$ \emph{H} $||$ C1), we measured the presence of a multitude of Landau level (LL) transitions, all with frequency dependence $\omega \propto \sqrt{H}$. We discuss the ramification of this observation for the surface and bulk properties of topological insulators. 
A.A. Schafgans, A.A. Taskin, Yoichi Ando, Xiao-Liang Qi, B.C. Chapler,   K.W. Post, and D.N. Basov
cond-mat.mes-hall