VIBRATIONAL DYNAMICS OF A STEPPED METALLIC SURFACE - STEP-EDGE PHONONS AND TERRACE SOFTENING ON NI(977)

Inelastic helium atom scattering has been used to measure the surface and step localized phonons on a stepped metallic surface, Ni(977). These time-of-flight measurements were carried out both perpendicular and parallel to the step direction. Surface phonon dispersion data collected across the steps show backfolding of the surface Rayleigh mode, and, most importantly, dramatic softening as compared to the forces present at the smooth Ni(111) surface. This softening suggests significant relaxation perpendicular to the step edge. Single-phonon scattering data collected along the step direction reveals the presence of two new step-edge localized modes, as well as the Rayleigh mode for this direction of the crystal. The Rayleigh mode here does not exhibit the notable softening that was found for the other direction. Novel in- and out-of-phase scattering measurements, with respect to the terraces, lead us to assign the new step induced modes as the two transversely polarized vibrations which propagate along the direction of the step edge. An analytic one-dimensional lattice model is proposed which well represents the dispersion data for these two step modes; its use allows us to determine the effective local force field in the two transverse directions with respect to the step edge. The findings reported herein shed new light on such topics as interface stability, crystal growth, and charge redistribution in the vicinity of well-characterized extended surface defects. © 1995 American Institute of Physics.