Generalized electronic susceptibility and charge density waves in transition metal dichalcogenides

Abstract

Lattice vibrations in 2H-TaSe 2 and 2H-NbSe 2 are investigated by one- and two-phonon Raman scattering in the normal and charge density wave (CDW) phases. The generalized electronic susceptibility is obtained from the two-phonon scattering intensity as a function of temperature, and with its use the temperature dependent energies of the Kohn anomaly mode and the soft CDW modes are calculated. The agreement with the directly obtained data from this experiment is excellent. In 2H-TaSe 2 four A g-modes and two B 1g-modes are observed in the commensurate CDW state as expected from the orthorhombic superlattice structure. The discommensuration in the incommensurate CDW phase gives the continuous change of the Raman spectra over the three CDW phases.