Résumé
Here we report on Landau level spectroscopy in magnetic fields up to 34 T performed on a thin film of the topological insulator Bi 2 Te 3 epitaxially grown on a BaF 2 substrate. The observed response is consistent with the picture of a direct-gap semiconductor in which charge carriers closely resemble massive Dirac particles. The fundamental band gap reaches E g = (175 ± 5) meV at low temperatures and it is not located on the trigonal axis, thus displaying either sixfold or twelvefold valley degeneracy. Interestingly, our magneto-optical data do not indicate any band inversion at the direct gap. This suggests that the fundamental band gap is relatively distant from the point where profound inversion exists and gives rise to the relativisticlike surface states of Bi 2 Te 3 .