Effect of impurities on the charge-density-wave dynamics in ${\mathrm{Ta}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Nb}}_{\mathrm{x}}$${\mathrm{S}}_3$ alloys

We report conductivity and dielectric-constant measurements on a series of orthorhombic ${\mathrm{Ta}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Nb}}_{\mathrm{x}}$ ${\mathrm{S}}_3$ alloys, with nominal concentrations between 0% and 0.3%, in the dc to 100-GHz frequency range. The Peierls transition observed at $T_P$=220 K in the pure specimens is smeared and suppressed in the alloys. The pinned–charge-density-wave mode, which occurs at the pinning frequency of ${\omega }_0$/2π∼5 GHz in the nominally pure specimen, increases with increasing dopant concentration at all temperatures. We analyzed the pinned mode in terms of a harmonic-oscillator response and found that the effective mass $m^{\mathrm{*}}$ and damping constant 1/τ are independent of the impurity concentration. We also discuss the temperature dependence of ${\omega }_0$ and τ and compare with the available theoretical descriptions of charge-density-wave dynamics.