Raman scattering spectra in the normal phase of 2H-NbSe2

Polarized Raman spectra have been measured on the layer-structure compound 2H-NbSe2 single crystal, in a temperature range from 15 to 300 K. The temperature dependence of phonon energies can be described by the Klemens model which assumes that a zone-centre phonon decays into two acoustical phonons with opposite momenta in the same acoustic branch. The low-energy broad peak in the Raman spectra was found to shift to lower energies with decreasing temperature in the normal phase. The results show that the broad peak can be considered as a two-phonon soft mode described by one soft phonon plus one normal phonon mode, with the vectors of the two phonons k-vector1+k-vektor2∼0