FLUCTUATION EFFECTS IN QUASI-ONE-DIMENSIONAL CONDUCTORS - OPTICAL PROBING OF THERMAL LATTICE FLUCTUATIONS

We have studied the effect of thermal lattice fluctuations on the opti cal properties of the charge-density wave (CDW) condensates K0.3MoO3 a nd (TaSe4)(2)I. From the temperature dependence of the CDW gap absorpt ion in the midinfrared frequency range, there is a strong indication o f the important role played by the thermal lattice fluctuation and zer o-point lattice motion. The latter removes the inverse square-root sin gularity expected for the case of a static distorted lattice. In fact, a considerable broadening (i.e., larger than k(B)T) of the subgap tai l absorption is found by increasing the temperature towards the transi tion temperature T-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 prope rties as disorder. Better agreement with the theory is obtained when t he contribution to the disorder parameter from phase phonon modes is a lso taken into account.