Effect of charge density wave fluctuations on the frequencies of optic phonons in 2H-TaSe₂ and -NbSe₂

The temperature dependence of the frequencies of the Raman active optic phonons between 200 and 250 cm-1 in 2H-TaSe2 and -NbSe2 has been measured for 2 K<T<300 K. The observed shifts are considerably larger than would be expected on the basis of measurements made on the isostructural semiconducting layer compound 2H-MoSe2. The anomalous temperature dependence in the charge density wave (CDW) state. The temperature dependence of the phonon frequencies for temperatures above the CDW phase transition suggests the presence of localized CDW distortions in the normal state. © 1978.