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.

Large surface charge density oscillations induced by subsurface phonon resonances.

A density functional perturbation theory study of Cu(111) surface dynamics and phonon-induced surface charge density (SCD) oscillations shows that the subsurface phonon resonances such as S3, first predicted by embedded-atom methods, trigger large SCD charge-density oscillations, thus explaining the large helium atom scattering intensity from the anomalous longitudinal resonance found in most metal surfaces. The strong coupling between certain phonons and SCD oscillations is shown to have implications in inelastic electron tunneling spectroscopy and other manifestations of electron-phonon interactions at metal surfaces.

Role of bulk and surface phonons in the decay of metal surface States.

We present a comprehensive theoretical investigation of the electron-phonon contribution to the lifetime broadening of the surface states on Cu(111) and Ag(111), in comparison with high-resolution photoemission results. The calculations, including electron and phonon states of the bulk and the surface, resolve the relative importance of the Rayleigh mode, being dominant for the lifetime at small hole binding energies. Including the electron-electron interaction, the theoretical results are in excellent agreement with the measured binding energy and temperature dependent lifetime broadening.