Vibrational dynamics of a stepped metallic surface: Step-edge phonons and terrace softening on Ni(977)

Abstract

Inelastic helium atom scattering has been used to measure the surface and step localized phonons on a stepped metallic surface, Ni͑977͒. 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 Ni͑111͒ 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.

Temperature dependence of atomic relaxation and vibrations for the vicinal Ni(977) surface: a molecular dynamics study

“…6(a) shows the PLSDP for the first three surfaces and bulk atoms at 300 K for Y points of surface Brillouin zone. The low-frequency part of the calculated spectrum for SC atoms shows a step-localized mode at 3.7 THz with mainly X and Z polarization, in agreement with the HAS experiment [3] for the surface Rayleigh modes at 3.6 THz. The step-localized mode is much softer compared to our calculated Rayleigh mode of the Ni (1 1 1) surface, which has a zone edge frequency of 4.3 THz in M point.…”

“…HAS experiment [3] showed two step induced modes, with the frequency of 1.15 THz and 1.62 THz, respectively. Our calculated results reveal a mode polarized in the x direction with the frequency of 1.30 THz, and another mode mainly polarized in the z direction with the frequency of 1.63 THz, which are in reasonable agreement with the experiments.…”

“…The dynamical properties of stepped surfaces have been studied by many surface science techniques. Using the inelastic He atom scattering technique, Witte et al [2] studied the effects of surface steps on the surface vibrational spectra of Cu(2 1 1) and Cu(5 1 1), and Niu et al [3] studied the dynamics of Ni(9 7 7) [3]. 0039 Various theoretical methods employ the slab model [4] to calculate the phonon spectrum and phonon dispersion curve of surfaces.…”

“…Since experimental data for surface phonon frequencies [3] and another theoretical calculation [9] are available on Ni(9 7 7), in this paper we use the MD method and MAEAM potential to calculate the temperature dependence of the surface relaxation, phonon spectrum, mean square displacement of atoms, and the layer structure factor for the stepped (9 7 7) surface of Ni up to 1700 K, which is only 60 K below the bulk melting point. Present calculation is carried out for much higher temperature than real space GreenÕs function approach [9].…”

Surface-scattering study of the interaction potential of He atoms with the step edges of the Cu(211) and Cu(511) vicinal surfaces

“…Recently, in the search for more detailed microscopic information on special dynamical features that might result from electronic modifications at step edges, Sibener and co-workers 30,31 and Witte et al 32 have investigated the steplocalized phonons on the Ni͑997͒ and the Cu͑211͒ and the Cu͑511͒ surfaces, respectively. In the latter study the step phonon modes could be clearly identified and their dispersion in k space along the two main symmetry directions could be traced out.…”