Landau-Level Spectroscopy of Relativistic Fermions with Low Fermi Velocity in the Bi2Te3 Three-Dimensional Topological Insulator

X-band microwave spectroscopy is applied to study the cyclotron resonance in Bi2Te3 exposed to ambient conditions. With its help, intraband transitions between Landau levels of relativistic fermions are observed. The Fermi velocity equals to 3260 m/s, which is much lower than has been reported in the literature for samples cleaved in vacuum. Simultaneous observation of bulk Shubnikov-de Haas oscillations by contactless microwave spectroscopy allows determination of the Fermi-level position. Occupation of topological surface states depends not only on bulk Fermi level but also on the surface band bending.