Temperature dependence of the charge-density-wave mass and relaxation time

We have investigated the temperature dependence of the effective mass and relaxation time associated with the dynamics of charge-density waves in pure (${\mathrm{TaSe}}_4$ ${)}_2$I and ${\mathrm{K}}_{0.3}$ ${\mathrm{MoO}}_3$ by conducting frequency-dependent conductivity measurements in the micrometer- and millimeter-wave spectral ranges. In both cases the magnitude and the temperature dependence of the effective mass is well described by a mean-field expression of the charge-density-wave dynamics. The temperature dependence of the relaxation time has also been evaluated, and it is contrasted with a semiphenomenological model assuming forward and backward scattering. The possible sources of disagreement between theory and experiment are discussed.