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

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.