Charge-density-wave transport in orthorhombic TaS3. I. Nonlinear conductivity

Dc conductivity measurements are reported on the linear chain compound Ta${\mathrm{S}}_3$ (orthorhombic form), which undergoes a phase transition to a charge-density-wave (CDW) state at $T_P=215\mathrm{}$ K. Strong resistive fluctuations, associated with the buildup of one-dimensional correlations, are observed above $T_P$. Below the transition nonlinear conductivity is observed when the electric field $E$ is larger than a threshold field $E_T$. $E_T$ is approximately 2 V/cm, and is weakly temperature dependent. The functional dependence of the conductivity $\sigma$ on $E$ is analyzed in terms of various models. A good agreement is found with Bardeen's tunneling model, which leads to a field-dependent conductivity $\sigma \mathrm{}\left(E\right)\mathrm{}\sim (1-\frac{E_T}{E})\exp (-\frac{E_0}{E})$, but Fleming's empirical formula also provides a good fit to the data. The low-threshold field and the strong field dependence indicate small pinning energies and large coherence and correlation lengths. At low temperatures the coherent response of the CDW condensate is absent, and we argue that, because of the loss of coupling between the CDW segments, a disordered CDW state develops. These observations are compared with those made on a similar linear chain compound, Nb${\mathrm{Se}}_3$.