Surface phonons and superstructures; Some applications

Abstract

Experiments using the technique of low energy electron diffraction (LEED) have shown that the periodic configuration of the atoms on some clean surfaces for a wide range of crystals is different from what is expected from the bulk configuration. A simple lattice dynamical calculation is presented showing that surface superstructures may exist even if interactions near the surface remain unchanged by the “bond-breaking” procedure. A calculation has been made for surface phonons at the (001) surface of some fcc and bcc transition metals. A model consisting of first and second neighbor central interactions together with angle bending forces is used — specific results have been obtained for surface phonons of Pt, Pd, Cu and Ag for fcc metals as well as Fe and W for bcc crystals. The agreement with neutron measurements for the bulk phonons is good. The (001) surface phonons for the above cited transition metals are obtained for the first time to our knowledge. The effect of variations of the interactions near the surface for Pt has been also investigated showing soft surface phonons for surface forces decreased by about 35% to 40% as compared to their bulk values. This model has been extended to study the vibrations of an adsorbed layer namely in the case concerning rare gazes (Ne, Xe,…). A similar attempt showing possible magnetic surface superstructure is also reported.