Electronic band structure of ${\mathrm{Sb}}_2{\mathrm{Te}}_3$

We report on Landau-level spectroscopy of an epitaxially grown thin film of the topological insulator ${\mathrm{Sb}}_2{\mathrm{Te}}_3$, complemented by ellipsometry and magnetotransport measurements. The observed response suggests that ${\mathrm{Sb}}_2{\mathrm{Te}}_3$ is a direct-gap semiconductor with the fundamental band gap located at the $\Gamma$ point or along the trigonal axis, and its width reaches $E_g=(190\pm 10)$ meV at low temperatures. Our data also indicate the presence of other low-energy extrema with a higher multiplicity in both the conduction and valence bands. The conclusions based on our experimental data are confronted with and to a great extent corroborated by the electronic band structure calculated using the $GW$ method.