Evidence for coupling between collective state and phonons in two-dimensional charge-density-wave systems

TL;DR

This study uses Raman scattering to explore the interaction between collective charge-density-wave states and phonons in two-dimensional rare-earth tellurides, revealing a strong coupling evidenced by phonon intensity scaling with the CDW gap.

Contribution

It provides experimental evidence of the coupling between CDW states and phonons, supported by first principles calculations and pressure-dependent measurements.

Findings

Phonon peaks correspond to Raman active modes in both undistorted and distorted lattices.

The phonon dispersion shows a Kohn anomaly driving the CDW transition.

Phonon mode intensities scale with the CDW-gap amplitude under pressure.

Abstract

We report on a Raman scattering investigation of the charge-density-wave (CDW), quasi two-dimensional rare-earth tri-tellurides $R$Te$_3$ ($R$= La, Ce, Pr, Nd, Sm, Gd and Dy) at ambient pressure, and of LaTe$_3$ and CeTe$_3$ under externally applied pressure. The observed phonon peaks can be ascribed to the Raman active modes for both the undistorted as well as the distorted lattice in the CDW state by means of a first principles calculation. The latter also predicts the Kohn anomaly in the phonon dispersion, driving the CDW transition. The integrated intensity of the two most prominent modes scales as a characteristic power of the CDW-gap amplitude upon compressing the lattice, which provides clear evidence for the tight coupling between the CDW condensate and the vibrational modes.