Sliding charge-density waves. II. ac properties

A classical model of a sliding charge-density wave (CDW) in a random pinning potential is used to determine the frequency-dependent conductivity ${\sigma }_E\left(\omega \right)$ in the presence of a dc electric field $E$ well above threshold. Specific interference features are predicted for both the in-phase and out-of-phase components of ${\sigma }_E\left(\omega \right)$. The contribution to the real part, ${\epsilon }_E\left(\omega \right)$, of the dielectric constant due to the pinning potential is shown to change sign, becoming negative as $\omega$ decreases. In the low-frequency limit the field dependence is const—${\epsilon }_E(\omega \to 0)\propto E^{-\frac{3}{2}}$. Voltage fluctuations observed with dc currents in the incommensurate CDW system Nb${\mathrm{Se}}_3$ are shown to be absent, to all orders of perturbation theory, in the infinite-volume limit.