A Raman study of the charge-density-wave state in A(0.3)MoO3 (A= K, Rb)

We report a comparative Raman spectroscopic study of the quasione-dimensional charge-density-wave (CDW) systems A0.3MoO3 (A = K,Rb). Temperature- and polarization-dependent experiments reveal charge-coupled vibrational Raman features. The strongly temperature-dependent collective amplitudon modes in the two materials differ by about 3 cm−1 , thus revealing the role of the alkali atom. We discuss the observed vibrational features in terms of the CDW ground state accompanied by a change in the crystal symmetry. A frequency-kink in some modes seen in K0.3MoO3 between T = 80 and 100 K supports the first-order lock-in transition, unlike the case of Rb0.3MoO3. 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 temperaturedependent CDW gap