Theory of surface phonons at metal surfaces: recent advances

Recent studies of the surface dynamics of Al(001) and Cu(111) based on density functional perturbation theory have substantiated the existence of subsurface optical phonon resonances of all three polarizations, thus confirming early predictions of the embedded-atom method. The hybridization of the shear-vertical optical resonance with the longitudinal acoustic phonon branch accounts for the ubiquitous anomalous acoustic resonance as an intrinsic feature of metal surfaces. The DFPT calculation of the phonon-induced surface charge density oscillations shows that helium atom scattering spectroscopy (HAS) can indeed probe subsurface resonances. This opens new perspectives to HAS for the measurement of subsurface phonon dispersion curves in thin films, as proved by recent HAS studies on Pb and Fe ultrathin films on copper. After discussing these recent advances, this paper briefly reviews other important trends of surface dynamics expressed in recent years.