Sliding charge-density waves. II. ac properties

Abstract

A classical model of a sliding charge-density wave (CDW) in a random pinning potential is used to determine the frequency-dependent conductivity ${\ensuremath{\sigma}}_{E}(\ensuremath{\omega})$ 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 ${\ensuremath{\sigma}}_{E}(\ensuremath{\omega})$. The contribution to the real part, ${\ensuremath{\epsilon}}_{E}(\ensuremath{\omega})$, of the dielectric constant due to the pinning potential is shown to change sign, becoming negative as $\ensuremath{\omega}$ decreases. In the low-frequency limit the field dependence is const---${\ensuremath{\epsilon}}_{E}(\ensuremath{\omega}\ensuremath{\rightarrow}0)\ensuremath{\propto}{E}^{\ensuremath{-}\frac{3}{2}}$. 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.