Optical spectroscopy study of charge density wave order in Sr$_{3}$Rh$_{4}$Sn$_{13}$ and (Sr$_{0.5}$Ca$_{0.5}$)$_{3}$Rh$_{4}$Sn$_{13}$

TL;DR

This study uses optical spectroscopy to investigate charge density wave transitions in Sr3Rh4Sn13 and its calcium-doped variant, revealing partial Fermi surface gapping and a strong coupling CDW gap.

Contribution

First optical spectroscopy analysis of CDW order in these compounds, showing large energy gaps and partial Fermi surface gapping, expanding understanding of CDW phenomena in this material class.

Findings

CDW energy gap observed in both compounds

Fermi surface is only partially gapped in the CDW state

The gap ratio 2Δ/kBT_CDW is approximately 13, indicating strong coupling

Abstract

We perform optical spectroscopy measurement across the charge density wave (CDW) phase transitions on single-crystal samples of Sr$_{3}$Rh$_{4}$Sn$_{13}$ and (Sr$_{0.5}$Ca$_{0.5}$)$_{3}$Rh$_{4}$Sn$_{13}$. Formation of CDW energy gap was clearly observed for both single-crystal samples when they undergo the phase transitions. The existence of a Drude component in $\sigma_1(\omega)$ below \TCDW indicates that the Fermi surface is only partially gapped in the CDW state. The obtained value of 2$\Delta$/K$_{B}$T$_{CDW}$ is roughly 13 for both Sr$_{3}$Rh$_{4}$Sn$_{13}$ and (Sr$_{0.5}$Ca$_{0.5}$)$_{3}$Rh$_{4}$Sn$_{13}$ compounds. The value is considerably larger than the mean-field value based on the weak-coupling BCS theory. The observed spectral feature in (Sr$_{x}$Ca$_{1-x}$)$_{3}$Rh$_{4}$Sn$_{13}$ resembles those seen in many other CDW systems.