Van Hove Singularity Arising from Mexican-Hat-Shaped Inverted Bands in the Topological Insulator Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S

TL;DR

This study investigates the optical properties of Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S, revealing a van Hove singularity linked to Mexican-hat-shaped inverted bands, which is significant for understanding strongly correlated phases in topological insulators.

Contribution

The paper identifies a van Hove singularity in a topological insulator due to Mexican-hat-shaped inverted bands, supported by optical measurements and first-principles calculations, highlighting a novel electronic feature.

Findings

No Drude response down to 30 cm$^{-1}$, confirming bulk insulation.

Observation of a sharp peak at 2200 cm$^{-1}$ at 5 K.

Peak attributed to van Hove singularity from Mexican-hat-shaped bands.

Abstract

The optical properties of Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_{2}$S, 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$^{-1}$, corroborating the excellent bulk-insulating property of this material. Intriguingly, we observe a sharp peak at about 2\,200~cm$^{-1}$ 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.