Fluctuation effects in quasi-one-dimensional conductors: Optical probing of thermal lattice fluctuations

We have studied the effect of thermal lattice fluctuations on the optical properties of the charge-density wave (CDW) condensates ${\mathrm{K}}_{0.3}$ ${\mathrm{MoO}}_3$ and (${\mathrm{TaSe}}_4$ ${)}_2$I. From the temperature dependence of the CDW gap absorption in the midinfrared frequency range, there is a strong indication of the important role played by the thermal lattice fluctuation and zero-point lattice motion. The latter removes the inverse square-root singularity expected for the case of a static distorted lattice. In fact, a considerable broadening (i.e., larger than ${\mathit{k}}_{\mathit{B}}$T) of the subgap tail absorption is found by increasing the temperature towards the transition temperature ${\mathit{T}}_{\mathrm{CDW}}$. We compare our experimental findings with the predictions of a theoretical model, which suggests that zero-point and thermal lattice motions have a similar effect on the electronic properties as disorder. Better agreement with the theory is obtained when the contribution to the disorder parameter from phase phonon modes is also taken into account.