Charge-density-wave transport in orthorhombic TaS3. III. Narrow-band "noise"

Narrow-band noise measurements are reported for the linear chain Peierls semiconductor, orthorhombic Ta${\mathrm{S}}_3$. In the nonlinear conductivity region, sharp narrow-band noise peaks are observed with several harmonics. The amplitudes of the peaks are temperature dependent, starting from zero just below the charge-density-wave (CDW) transition at $T_P\sim 215$ K, and remaining approximately constant below $T=170\mathrm{}$ K. Between $T_P$ and approximately 100 K, where both the field- and the frequency-dependent conductivities suggest highly coherent response of the CDW condensate, the noise spectrum is simple (fundamental and harmonics). At low temperatures the noise spectrum becomes quite complicated, with many fundamental and harmonic peaks present, due to a loss of the transverse coherence in the material and indicative of a disordered CDW state. At intermediate temperatures a linear relationship is found between the excess CDW current and the fundamental noise frequency. Analysis in terms of coherent motion of the CDW indicates that the characteristic distance associated with the narrow-band noise is approximately equal to one CDW wave-length $\lambda$ and that pinning is dominated by impurities rather than by commensurability. A comparison of the data is made with the predictions of the classical model of CDW transport and with Bardeen's theory of coherent current oscillations. The noise observations are compared to and contrasted with related measurements performed on the similar CDW material Nb${\mathrm{Se}}_3$.