Complete excitation spectrum of charge-density waves: Optical experiments onK0.3MoO3

Frequency-dependent conductivity measurements are reported in the charge-density-wave state of ${\mathrm{K}}_{0.3}$ ${\mathrm{MoO}}_3$ and ${\mathrm{K}}_{0.3}$ ${\mathrm{Mo}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{W}}_{\mathit{x}}$ ${\mathrm{O}}_3$ alloys in the radio-frequency, microwave, millimeter-wave, and infrared spectral range. We identify several distinct features associated with the electrodynamics of the ground state. The dynamics of internal deformations of the collective mode, appearing at radio frequencies, together with the pinned-mode resonance at millimeter-wave frequencies are analyzed in detail and we evaluate the parameters that characterize these modes. We also identify a resonance in the far-infrared spectral range and suggest that it is due to bound charge-density-wave states. We account for the phonon spectra in terms of a phase-phonon description and also analyze the optical response due to single-particle excitations across the gap.