High-frequency phonon modes on stepped and kinked Cu surfaces: Experimentsand theory

Using electron energy loss spectroscopy, we have found step-localized vibra tional modes with frequencies higher than those of the bulk phonons on Cu(5 32) and Cu(211). From structural and lattice dynamical calculations based o n many-body interaction potentials, we trace the origin of these modes to l ocal effects involving surface relaxations and resulting stiffening of forc e constants between the step (or kink-site) atom and its "bulk" nearest nei ghbor. Such modes are not observed on Cu(511) and Cu(17,1,1), although they are predicted in the theoretical calculations. We discuss the impact of th ese results on systems with bond-length-bond-order correlation.