A Raman study of the Charge-Density-Wave State in A$_{0.3}$MoO$_3$ (A = K,Rb)

TL;DR

This study uses Raman spectroscopy to compare charge-density-wave states in K and Rb variants of A$_{0.3}$MoO$_3$, revealing temperature-dependent vibrational features and phase transitions influenced by alkali atoms.

Contribution

It provides the first comparative Raman analysis of A$_{0.3}$MoO$_3$ with different alkali metals, highlighting the role of alkali atoms in charge-density-wave properties and phase transitions.

Findings

Charge-coupled vibrational Raman features depend on temperature and polarization.

The amplitudon mode differs by about 3 cm$^{-1}$ between K and Rb compounds.

Evidence of a first-order lock-in transition between 80 K and 100 K.

Abstract

We report a comparative Raman spectroscopic study of the quasi-one-dimensional charge-density-wave systems \ab (A = K, Rb). The temperature and polarization dependent experiments reveal charge-coupled vibrational Raman features. The strongly temperature-dependent collective amplitudon mode in both materials differ by about 3 cm, thus revealing the role of alkali atom. We discus the observed vibrational features in terms of charge-density-wave ground state accompanied by change in the crystal symmetry. A frequency-kink in some modes seen in \bb between T = 80 K and 100 K supports the first-order lock-in transition, unlike \rb. The unusually sharp Raman lines(limited by the instrumental response) at very low temperatures and their temperature evolution suggests that the decay of the low energy phonons is strongly influenced by the presence of the temperature dependent charge density wave gap.