Infrared spectroscopy study of kagome material CsTi$_3$Bi$_5$

TL;DR

This study uses optical spectroscopy and first-principles calculations to explore the electronic properties of CsTi$_3$Bi$_5$, revealing the absence of charge density wave instability and highlighting the role of van Hove singularity in related kagome materials.

Contribution

It provides the first comprehensive optical and theoretical analysis of CsTi$_3$Bi$_5$, demonstrating the absence of CDW and linking electronic features to van Hove singularity effects.

Findings

CsTi$_3$Bi$_5$ lacks CDW instability unlike CsV$_3$Sb$_5$.

Reduced carrier density in CsTi$_3$Bi$_5$ explains the absence of CDW.

Evidence of flat bands in CsTi$_3$Bi$_5$ from low-lying absorption features.

Abstract

The kagome material CsTi$_3$Bi$_5$, which is isostructural to the extensively studied charge density wave (CDW) compound CsV$_3$Sb$_5$, exhibits intriguing electronic features within its two-dimensional kagome lattices of titanium atoms. Here, we perform optical spectroscopic measurements together with the first-principles calculations on single-crystalline CsTi$_3$Bi$_5$ to investigate its electronic properties comprehensively. It is found that the overall optical spectra are very similar to those of CsV$_3$Sb$_5$, but the existence of CDW instability is ruled out in CsTi$_3$Bi$_5$. Via careful comparison to the optical responses of CsV$_3$Sb$_5$, we attribute this difference to a significant reduction in the itinerant carrier density of CsTi$_3$Bi$_5$, which is associated with the absence of van Hove singularity near the Fermi level at $M$ point. This result supports the scenario that the CDW in CsV$_3$Sb$_5$ is driven by the nesting of van Hove singularity. Additionally, we unveil some exotic low-lying absorption features, which provide clear evidence of flat bands in CsTi$_3$Bi$_5$. Our findings contribute to a deeper understanding of exotic phenomena in CsTi$_3$Bi$_5$ and provide valuable insights into the role of van Hove singularity in CsV$_3$Sb$_5$.