Complete excitation spectrum for a charge-density-wave system

We report measurements of the far-infrared (FIR) reflectance of the charge-density-wave (CDW) conductor ${\left(\mathrm{T}\mathrm{a}{\mathrm{Se}}_4\right)}_2$I. In the CDW state, and for incident electric field polarized parallel to the crystal chain direction, the associated FIR conductivity shows a giant resonance at 38 ${\mathrm{cm}}^{-1\mathrm{}}$. Our results, when combined with previous measurements at lower and higher frequencies, provide the first consistent and complete excitation spectrum of a CDW system. We propose that a giant FIR mode distinct from the pinned CDW mode is a generic feature of CDW systems.