HELIUM-ATOM-SCATTERING STUDY OF THE DISPERSION-CURVES OF STEP-LOCALIZED PHONONS ON CU(211) AND CU(511)

The surface phonon dispersion curves of two different regularly steppe d vicinal copper surfaces have been measured using high resolution ine lastic helium-atom scattering. The Cu(211) and Cu(511) surfaces were c hosen since they have comparable step distances but different terrace orientations (111) and (100), respectively. The Rayleigh mode was meas ured over the entire Brillouin zone along azimuths parallel and perpen dicular to the step edge directions. For both surfaces backfolding of the Rayleigh mode was observed along the direction perpendicular to th e step edges, while parallel to the step edges, a step localized mode with optical character and a weak dispersion were also found. For the Cu(511) surface, an additional longitudinal mode and an energy gap for the Rayleigh mode at the zone boundary could be identifed, whereas on Cu(211), an additional horizontal mode could be identified. The low z one boundary energy observed for the latter mode can be related to the relaxation of the step atoms. The experimental data can be well expla ined by theoretical slab calculations in the framework of a single for ce constant model.