Resonances in the surface-phonon spectrum of an sp-bonded metal

We report He-atom time-of-flight measurements and microscopic calculations of surface-phonon spectral densities and dispersion curves, revealing the presence of anomalous resonances in the phonon spectrum for low-index surfaces of Al. These resonances are similar to those observed in d-electron metals, for which they have been attributed to sp-d hybridization effects. For sp-bonded Al the resonances are not related to a softening of a single force constant, but rather to cumulative effects of changes in the entire surface force field.