Phonon dispersion at the Be(0001) surface

Abstract
Measurements of surface-phonon dispersion at the Be(0001) surface are reported that are found to differ significantly from the predictions of an accurate model of the truncated bulk: the sign of the Rayleigh wave dispersion along the zone boundary is reversed in theory and experiment. The measured dispersion implies a reduction in the magnitude of noncentral forces at the surface. Such a reduction is compatible with the electronic structure of the Be(0001) surface, which is more free-electron-like than that of bulk Be due to a high density of surface states. We conclude that the properties of the surface states dominate the dynamics of surface atoms.