Theory of surface vibrations in epitaxial thin films

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Abstract
Recent inelastic helium atom scattering measurements of the phonon dispersion curves of epitaxial thin films (2–30 ML) of four different types of systems are analyzed within the framework of a force constant model. Whereas Na/Cu(001) shows nearly flat dispersionless modes with frequencies at the Γ¯ point that follow the open standing wave sequence, the other systems Pb/Cu(111), Ar(Kr)/Ag(111), and KBr/NaCl(001) reveal a more complex behavior. These qualitative differences can be explained in terms of a simple linear chain model in which the behavior is determined by the strength of the force constant coupling of the thin films to the substrate relative to the force constant within the films. Detailed slab calculations for Na/Cu(001) confirm this model and indicate that the observed flat phonon dispersion curves are related to the strength of the interface interaction and bcc structure of the epitaxially grown Na thin films. © 1996 The American Physical Society.