Infrared Spectroscopy Study Of The Novel Van Hove Singularity In A Topological Insulator

Owing to the fascinating physics exhibited by flatband systems,the search for materials with flat bands near E_F is highly motivated.So far,flat bands have been found in many systems,such as high-T_c cuprates,twisted bilayer graphene,and heavy fermion compounds.Topological insulators have also been predicted by theoretical calculations to possess flat bands in the bulk.In recent years,a high-quality topological insulator Sn-doped Bi_1.1Sb_0.9Te₂S has been synthesized by Cava's group.This material has no bulk states with E_F in the bulk band gap,providing a perfect platform for optical studies of the possible flat bands.We used a Bruker Vertex 80V Fourier transform infrared spectrometer equipped with a home-made in situ gold evaporation setup to measure the reflectivity of Sn-doped Bi_1.1Sb_0.9Te₂S,the most bulk-insulating topological insulator thus far,at 16 different temperatures over a broad frequency range from 30-50000cm^-1.No Drude response is detected in the low-frequency range,in good agreement with the excellent bulkinsulating property of this material.Intriguingly,we observe a sharp peak at about 2200 cm^-1in the optical conductivity at 5 K,which can not be described by a Lorentzian oscillator.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 may not only account for the mechanism of the emergent quantum phases in topological insulators,but also pave a new way to the realization of topological superconductors.