Geometric Structure, Pseudoband Gaps, and Surface Vibrational Resonances on Metal Surfaces

摘要

It is shown by surface lattice dynamics that a new type of surface vibrational resonance arises in those frequency regions where there is a strong depletion in the bulk phonon density of states. The presence of these pseudo band gaps is due to higher Fourier components in the phonon dispersion relations introduced by the particular coordination of atoms in layers parallel to the surface. A quantitative analysis based on surface lattice dynamics of the recorded electron energy loss spectra of Cu and Ni suggests that the outermost surface interlayer force constant is rather close to the bulk value. This romance is found to exist throughout the X direction and makes an avoided crossing with a resonance derived from a band gap at the X point. An explanation is given for the disappearance of the divergent van Hove singularities in the projected bulk density of states upon projection on a surface layer.