Raman-Spectroscopy Of Soft Modes At Charge-Density-Wave Phase-Transition In 2h-Nbse2

The Raman spectra of the layer-structure compounds $2H\\ensuremath{-}\\mathrm{Nb}{\\mathrm{Se}}_{2}$ and $2H\\ensuremath{-}\\mathrm{Ta}{\\mathrm{Se}}_{2}$ have been measured between 2 and 300 K. A low-energy peak in the Raman spectrum is found to soften at the charge-density-wave phase transition. This peak is attributed to the charge-density-wave amplitude mode. The intensity and energy of this soft mode have been successfully compared with mean-field treatments of the charge-density-wave phase transition.