LAUSR

The optical properties of Sn-doped Bi1.1Sb0.9Te2S, the most bulk-insulating topological insulator thus far, have been examined at different temperatures over a broad frequency range. No Drude response is detected in the low-frequency range down to 30 cm, corroborating the excellent bulkinsulating property of this material. Intriguingly, we observe a sharp peak at about 2 200 cm in the optical conductivity at 5 K. Further quantitative analyses of the line shape and temperature dependence of this sharp peak, in combination with first-principles calculations, suggest that it corresponds to a van Hove singularity arising from Mexican-hat-shaped inverted bands. Such a van Hove singularity is a pivotal ingredient of various strongly correlated phases.