Extended phonon collapse in the charge-density-wave compound NbSe2

We investigated the phonon softening in the charge density wave compound NbSe2 using the high-resolution hard inelastic X-ray scattering beamline 30-ID-C at the Advanced Photon Source, Argonne National Laboratory. The acoustic Σ1 phonon branch was measured from the zone center Γ to the M point at temperatures between 250 K and 8 K across the CDW transition at TCDW=33 K. Density functional theory calculations for the lattice dynamical properties which predict an extended phonon breakdown are used to analyze the detailed nature of the softening phonon branch.