Shifted surface-phonon anomaly in 2H-TaSe2

A temperature-dependent soft-mode behavior is found in the surface–Rayleigh-phonon dispersion curve of 2H-${\mathrm{TaSe}}_2$ by means of inelastic He-atom scattering. The deepest anomaly occurs about 10% below the charge-density-wave onset temperature ${\mathrm{T}}_{\mathrm{o}}$=123 K and is surprisingly located at one-half of the reduced Brillouin zone, unlike the bulk longitudinal-acoustic anomaly which occurs at two-thirds of the zone.