Origin of the dielectric relaxation frequency in sliding-charge-density-wave systems

The charge-density-wave (CDW) dielectric relaxation frequency, which has been found to decrease at low temperatures according to an Arrhenius behavior in ${\left(\mathrm{T}\mathrm{a}{\mathrm{Se}}_4\right)}_2$I and ${\mathrm{K}}_{0.3}$Mo${\mathrm{O}}_3$, is identified with the ac-dc decoupling frequency previously seen in Ta${\mathrm{S}}_3$ and Nb${\mathrm{Se}}_3$. A simple model is proposed based upon normal carrier screening of the CDW polarization created near randomly distributed "strong-pinning" centers within the bulk crystal. Relaxation frequencies calculated in terms of this model agree precisely with experiment. A wide range of phenomena associated with the nature of the threshold field, coherent current oscillations, and high-frequency ac response appear to have natural explanations within this context.