Ultrahigh magnetic field spectroscopy reveals the band structure of the three-dimensional topological insulator Bi2Se3

We have investigated the band structure at the Gamma point of the three-dimensional topological insulator Bi2Se3 using magnetospectroscopy over a wide range of energies (0.55-2.2 eV) and in ultrahigh magnetic fields up to 150 T. At high energies (E > 0.6 eV) the parabolic approximation for the massive Dirac fermions breaks down and the Landau-level dispersion becomes nonlinear. At higher energies around 0.99 and 1.6 eV, additional strong absorptions are observed with temperature and magnetic field dependences which suggest that they originate from higher band gaps. Spin-orbit splittings for the further lying conduction and valence bands are found to be 0.196 and 0.264 eV.