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 K0.3{MoO}₃$ (blue bronze) and (${TaSe}₄)₂I in the conducting phase (above T3D=183 K and 263 K, respectively, but below the mean-field temperature TMF) have been measured over a broad frequency range, 1--10⁵ cm^-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 cm^-1 for the electric field E{→} parallel to the chains, the direction along which the charge-density wave develops below the Peierls transition temperature T3D. 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 TMF but above T3D, the temperature where three-dimensional long-range order develops.