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lead metallic thin films monolayers phonon dispersion relations rayleigh scattering surface phonons surface structure time of flight spectra helium-atom scattering ultrathin metallic-films lattice-dynamics lead films he-scattering density vibrations hydrogen growth w(110)
Abstract:
Thin films of lead have been grown on the Cu(111) surface and their structure and phonon dispersion curves have been investigated with high-resolution helium atom scattering. The diffraction pattern of the first monolayer (ML) indicates a p(4x4) structure. Films consisting of N=3, 4, 5, and bulklike 50 ML all have a regular (1x1) structure with the same lattice constant as the bulk and small superimposed p(4x4) peaks for the thinnest films. The time-of-flight spectra reveal an unusually large number of inelastic peaks for all the films. The results have been analyzed in terms of dispersion curves which exhibit more than 1/2 the total number of expected two N modes active in the planar scattering geometry. Dispersion curves for an unannealed 5 ML film are also reported. A force-constant model fitted to the bulk dispersion curves can only qualitatively reproduce some of the data but help to explain some features. Along the Gamma over bar-K over bar direction, a Kohn anomaly is identified for the 3, 4, and 5 ML films at nearly the same wave vector as in the bulk along the equivalent direction.
