Field and frequency dependence of charge-density-wave conduction in NbSe3

Abstract

Frequency ($\omega$) - and field ($E$)-dependent conductivity ($\sigma$) measurements are reported in both charge-density-wave (CDW) states of the linear-chain compound Nb${\mathrm{Se}}_3$. There is a direct scaling between the observed $E$ and $\omega$ dependence for parameters corresponding to $E>\mathrm{}2\mathrm{}{E}_T$, where $E_T$ is the threshold field for the onset of nonlinear conductivity. The functional form of $\sigma \mathrm{}\left(E\right)\mathrm{}$ is in agreement with a tunneling model. In contrast to the threshold field, there is no threshold frequency for the onset of frequency-dependent conductivity. This is accounted for by a contribution to $\sigma \left(\omega \right)$ from the excitation of oscillations of the pinned CDW at low frequencies. Experiments involving the combined application of dc and ac fields $V=V_0+V_{1}cos\omega t$ do not show evidence for photon-assisted tunneling, and only the classical limits of the tunneling formalism are observed. We discuss these observations and suggest reasons for the absence of quantum effects when both ac and dc fields are present. Various models of CDW transport are compared.