Effect of disorder in the charge-density-wave compounds LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16) as revealed by optical spectroscopy

TL;DR

This study uses optical spectroscopy to investigate charge density wave gaps and the impact of disorder in LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ compounds, revealing high-energy gaps and the significant role of Te vacancies in low-energy excitations.

Contribution

It demonstrates that disorder and Te vacancies significantly influence low-energy charge excitations in rare-earth ditellurides, contrasting with tri-tellurides, and applies the localization modified Drude model to interpret spectral features.

Findings

Charge density wave gaps are at high energies (~8500 cm$^{-1}$).

Te vacancies and disorder affect low-energy excitations distinctly from tri-tellurides.

Low-temperature spectra differ from a fully gapped semiconducting CDW state.

Abstract

We present optical spectroscopy measurements on rare-earth ditelluride single crystals of LaTe$_{1.95}$ and CeTe$_{1.95-x}$Se$_x$ (x=0 and 0.16). The measurements reveal formation of charge density wave energy gaps at rather high energy levels, e.g. 2$\Delta\sim$ 8500 \cm for LaTe$_{1.95}$, and 6800 \cm for CeTe$_{1.95}$. More strikingly, the study reveals that, different from the rare-earth tri-tellurides, the Te vacancies and disorder effect play a key role in the low-energy charge excitations of ditelluride systems. Although an eminent peak is observed between 800 and 1500 \cm in conductivity spectra for LaTe$_{1.95}$, and CeTe$_{1.95-x}$Se$_x$ (x=0. 0.16), our analysis indicates that it could not be attributed to the formation of a small energy gap, instead it could be well accounted for by the localization modified Drude model. Our study also indicates that the low-tempreature optical spectroscopic features are distinctly different from a semiconducting CDW state with entirely gapped Fermi surfaces.