Charge-density-wave motion in NbSe3. II. Dynamical properties

We have studied the quasiperiodic noise generated in Nb${\mathrm{Se}}_3$ above the critical electric field where the charge-density waves (CDW's) are depinned. The Fourier spectrum of the noise can be totally described with three fundamental frequencies and their harmonics. At high electric fields the fundamental frequencies are still observable at 100 MHz. We have performed interferences between these frequencies in the noise and an external rf field as measured in the low-frequency $\frac{\mathrm{dV}}{\mathrm{dI}}$ characteristic. With the model where the phase of the CDW is described as an overdamped oscillator we explain why we observe an increase in the differential resistance of the periodic frequencies. We explain also that, in spite of the great number of domains in the sample, a coherent signal is observed because of the self-synchronization of all the domains at a given frequency. We show that the periodic frequency has a linear variation with the current carried by the CDW. The inverse of this slope gives the number of electrons in the band affected by the CDW gap. We find that this number is the same for each CDW in Nb${\mathrm{Se}}_3$ and in excellent agreement with the number of electrons which can be obtained by simple band calculations.