Optical phonons and the soft mode in 2H-NbSe2

We present an investigation of the lattice dynamics of the charge density wave (CDW) compound 2H-NbSe2. We analyze the precise nature of the wave vector-dependent electron-phonon coupling (EPC) and derive the bare dispersion of the CDW soft phonon mode using inelastic x-ray scattering combined with ab initio calculations. Experimentally, phonon modes along the Gamma - M line, i.e., q=(h, 0,0), with 0 <= h <= 0.5 and the same longitudinal symmetry (Sigma(1)) as the CDW soft mode, were investigated up to 32 meV. In agreement with our calculations, we observe significant EPC in the optic modes at h <= 0.2. We analyze the EPC in the optic, as well as acoustic, mode and show that the q dependences stem from scattering processes between two bands at the Fermi surface that both have a Nb 4d character. Finally, we demonstrate that the soft mode dispersion at T=33 K (=T-CDW) can be well described on the basis of a strongly q-dependent EPC matrix element and an acousticlike bare phonon dispersion in agreement with observations near room temperature.