Raman spectroscopy of soft modes at the charge-density-wave phase transition in 2H-NbSe2

The Raman spectra of the layer-structure compounds 2H-NbSe2 and 2H-TaSe2 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. © 1976 The American Physical Society.