Fluctuation effects on the electrodynamics of quasi-one-dimensional conductors above the charge-density-wave transition

The complex conductivity spectra of the quasi-one-dimensional compounds ${\mathrm{K}}_{0.3}$ ${\mathrm{MoO}}_3$ (blue bronze) and (${\mathrm{TaSe}}_4$ ${)}_2$I in the conducting phase (above ${\mathit{T}}_{3\mathrm{D}}$=183 K and 263 K, respectively, but below the mean-field temperature ${\mathit{T}}^{\mathrm{MF}}$) have been measured over a broad frequency range, 1–${10}^5$ ${\mathrm{cm}}^{\mathrm{-}1}$, using a combination of different spectroscopic techniques; we have also investigated the effects of disorder. Clearly pronounced excitations are discovered in the spectra below 50 ${\mathrm{cm}}^{\mathrm{-}1}$ for the electric field E→ parallel to the chains, the direction along which the charge-density wave develops below the Peierls transition temperature ${\mathit{T}}_{3\mathrm{D}}$. We associate these excitations with charge-density-wave fluctuations that exist even at room temperature and result in a collective contribution to the conductivity. For the transverse polarization, E→ perpendicular to the chains, no such low-frequency excitations are evident in the conductivity spectra. These measurements give clear evidence for important deviations from conventional metallic behavior in the fluctuating region below ${\mathit{T}}^{\mathrm{MF}}$ but above ${\mathit{T}}_{3\mathrm{D}}$, the temperature where three-dimensional long-range order develops.