Charge-density-wave dynamics in (Ta1−xNbxSe4)2I alloys

Frequency-dependent conductivity and phase-resolved harmonic-mixing studies are reported on alloys of the linear-chain compound (${\mathrm{TaSe}}_4$ ${)}_2$I with Nb in the microwave and millimeter-wave spectral range. The frequency-dependent conductivity is evaluated in the charge-density-wave (CDW) state, and the collective-mode response is analyzed in terms of a harmonic-oscillator response with an effective mass ${\mathit{m}}^{\mathrm{*}}$, a pinning frequency ${\mathrm{\omega }}_0$, and relaxation time ${\mathrm{\tau }}^{\mathrm{*}}$ as parameters. We find that the magnitude, temperature, and concentration dependence of the effective mass ${\mathit{m}}^{\mathrm{*}}$ is well described in terms of the mean-field description of the dynamical CDW response. The pinning frequency ${\mathrm{\omega }}_0$ increases with increasing impurity concentration x, and the concentration dependence is suggestive of strong-impurity pinning. We also evaluate the temperature and concentration dependence of the relaxation time ${\mathrm{\tau }}^{\mathrm{*}}$, and compare this parameter with various models.