Burl M. Hall

Surface vibrations on Ni(110): The role of surface stress

Using inelastic electron scattering we have determined the dispersion of surface phonons on the clean Ni(110) surface along the [001] ( Γ ¯ Y ¯ ), the [ 1 1 ¯ 0 ] ( Γ ¯ X ¯ ), and the [ 1 1 ¯ 1 ] ( Γ ¯ S ¯ ) direction. A lattice dynamical calculation which takes surface relaxation into account shows good agreement with the experimental results for the resonance mode near Γ ¯ and for a longitudinal resonance along the [ 1 1 ¯ 0 ] direction when the interlayer surface force constants are scaled in relation to the experimentally determined lattice relaxation. Substantial deviations from the experimental data for the vertically polarized surfaces modes near X ¯ and Y ¯ remain however. They are attributed to the presence of (an anisotropic) surface stress on the Ni(110) surface and good overall agreement with the experimental results is found when a force field taking surface stress into account is used. With the help of the eigenvectors from the lattice dynamical model and using a fully dynamical theory for the inelastic electron scattering we have also calculated the spectra for the Y ¯ and X ¯ point and find good agreement with the experimental results at different impact energies. It is shown that bulk modes can contribute significantly to the spectrum even when one has no particular enhancement of the amplitude near the surface.

Lattice Dynamics of Rare Gas Multilayers on the Ag(111) Surface. Theory and Experiment.

We discuss the lattice dynamics of argon, krypton, and xenon overlayers on the Ag(111) surface. We consider monolayer, bilayer, trilayer and 25 layer films of each of these adsorbates. Data are also presented on the dispersion relations of selected branches of the phonon spectra of these overlayers. The data have been obtained by the method of angle‐resolved inelastic He scattering. Several models of the lattice dynamics are compared with the data. It is concluded that the gas phase potentials proposed by Barker and co‐workers prove suitable for a description of lateral interactions between the adsorbates, within the accuracy of the available data, provided that the phonon spectra are calculated for a lattice with the experimentally determined lattice constants.

Anharmonic linewidth broadening of surface phonons

Abstract The first measurement of the temperature-dependent intrinsic linewidth of a surface phonon is reported. The linewidth of the vertically polarized L1-surface phonon of rare gas bilayer crystals is found to increase linearly with temperature. Anharmonic linewidths calculated by inclusion of cubic terms in the expansion of the rare gas pair potentials are in accord with the experimental data, indicating two-phonon processes as the cause of broadening.

Calculation of high resolution electron and helium energy loss cross sections using surface vibration spectral densities

Abstract It is shown that vibrational eigenvectors (outputs of a slab calculation) are not required for the calculation of high resolution electron energy loss spectra (EELS) or helium energy loss spectra. Instead, vibrational spectral densities (outputs of a Greens function calculation) may be input directly to such calculations. This facilitates the treatment of scattering from bulk phonons as well as from surface phonons.

The multiple scattering theory of high resolution electron energy loss spectroscopy

Abstract The multiple scattering theory of High Resolution Electron Energy Loss Spectroscopy has been developed in the last decade. It has proved to be a very powerful tool in the study of surface lattice dynamics, since it predicts where the hard to observe longitudinally polarized surface modes can be seen. It is the purpose of this article to provide a brief review of the theory, to sketch the basic ingredients needed to perform calculations of the scattering cross sections, and to show some typical comparisons of theory and experiment.

Theoretical Studies of Spin Dependent Scatterings of Electrons from Ferromagnetic Surfaces and Ultra Thin Films

We summarize results of our recent theoretical studies of spin dependent scattering of electrons from ferromagnetic surfaces. We obtain an excellent account of both the energy and angle variation of the exchange asymmetry reported by Waller and Gradmann in their SPLEED study of the Fe(110) surface, and of spin dependent asymmetries in the transmission of photoelectrons through an ultra thin film of Fe on Cu(100). Potentials supplied by Fu and Freeman enable us to account for the data, with use of ground state potentials generated by ab initio methods.