Influence of Surface Force-Constant Changes on Surface-Mode Frequencies

Investigations of the dynamical properties of metallic crystals with free surfaces have been performed on the basis of a Born---von Karman model with atomic interactions up to second nearest neighbors including noncentral forces. The effect of a variation of the force constants near the surface on the surface-mode dispersion relations is analyzed. The calculations were based on a nonstandard Green's-function technique. Results are presented for the (100) surface of nickel. Surface modes are shown to exist below and above the cyclic frequency band as well as in a gap within the bulk region. The consequences of the fulfillment of the conditions of rotational invariance for the surface-mode dispersion relations are demonstrated.