Abstract
Inelastic helium atom scattering has been used to measure the surface phonon dispersion curves for the (001) face of the ordered phase of Cu 3Au along the 〈100〉 (i.e., γ − M ′) direction. We report the spectroscopic observation of two surface phonon modes on this fcc alloy, and present a detailed description of the scattering instrument that was used for making these measurements. The lower-energy surface phonon mode, the Rayleigh wave, has an energy of 7.1 ± 0.5 meV at M̄'. The higher-lying feature is an optical mode with an energy of 12.5 ± 1.0 meV, which shows little dispersion across the surface Brillouin zone. This phonon mode might be interpreted as a folded Rayleigh mode. The experimentally measured dispersion curves do not agree with those generated by a lattice dynamical slab calculation which uses a pair potential force-field that successfully models the bulk vibrations of the ordered alloy. The best fit to our experimental data indicates that the force constant between the first and second layer Cu atoms needs to be stiffened by ∼ 20% with respect to the corresponding bulk value.
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