Charge-density-wave transport in orthorhombic TaS3. II. Frequency-dependent conductivity

Frequency- ($\omega$) dependent conductivity measurements are reported on the linear-chain compound orthorhombic Ta${\mathrm{S}}_3$. A weak frequency dependence and positive dielectric constant are observed due to one-dimensional fluctuations of charge-density waves (CDW's) above the Peierls transition at $T_P=215\mathrm{}$ K. Below the phase transition the conductivity strongly increases with increasing $\omega$, and the dielectric constant is of the order of ${10}^7$. The small characteristic energies obtained from the observed $\omega$ dependence give direct evidence for the collective mode. When interpreted in terms of a classical model of the pinned CDW, $\sigma \left(\omega \right)$ indicates a strongly overdamped response. A tunneling model developed by Bardeen also leads to a consistent description of both $\sigma \left(\omega \right)$ and the electric-field- ($E$) dependent dc conductivity $\sigma \mathrm{}\left(E\right)\mathrm{}$. In this model the frequency-dependent response is due to both a tunneling contribution and the response of the pinned mode. Below about 130 K the observed frequency dependence suggests the development of a disordered CDW state. These observations are compared with experiments performed on the linear-chain compound Nb${\mathrm{Se}}_3$.