Shifted Surface-Phonon Anomaly in 2H-TASE2

A temperature-dependent soft-mode behavior is found in the surface Rayleigh-phonon dispersion curve of 2H-TaSe2 by means of inelastic He-atom scattering. The deepest anomaly occurs about 10% below the charge-density-wave onset temperature To=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. © 1988 The American Physical Society.