The optical properties of Sn-doped ${\mathrm{Bi}}_{1.1}{\mathrm{Sb}}_{0.9}{\mathrm{Te}}_2\mathrm{S}$, 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 $30{\mathrm{cm}}^{-1}$, corroborating the excellent bulk-insulating property of this material. Intriguingly, we observe a sharp peak at about $2200{\mathrm{cm}}^{-1}$ 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.

中文翻译：

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Van Hove奇异性是由掺杂Sn的Bi1.1Sb0.9Te2S拓扑绝缘体中的墨西哥帽形反向带引起的

掺锡的光学性质 ${双}_{1.1}{锑}_{0.9}{特}_2\mathrm{小号}$迄今为止，最绝缘的拓扑绝缘子已经在很宽的频率范围内的不同温度下进行了检查。在低频范围至$30{\mathrm{厘米}}^{-\mathrm{1个}}$，证实了这种材料出色的整体绝缘性能。有趣的是，我们在$2200{\mathrm{厘米}}^{-\mathrm{1个}}$在5 K时的光导率中。对这个尖峰的线形和温度依赖性的进一步定量分析，与第一性原理的计算相结合，表明它对应于由墨西哥帽形反向谱带引起的van Hove奇异性。这种van Hove奇异性是各种强相关相位的关键组成部分。