Surface phonons in Si(111) + H(1×1)

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Abstract
We present an experimental and theoretical study of the dispersion curves for vibrations on the Si(111)+H(1×1) surface. In order to explain the dispersion of the Rayleigh wave measured by He-atom scattering, we have calculated the surface phonon dispersion curves along the boundaries of the irreducible part of the surface Brillouin zone in the framework of the bond charge model for a slab of 22 atomic planes. By fitting the force constants related to the stretching and bending frequencies of the Si-H bond on the surface, the experimental Rayleigh mode is well reproduced. The good agreement includes the rather flat part along the zone boundary, where the frequency is considerably softened with respect to the bulk band. Finally, on the basis of our calculations we can explain the longitudinal-optical surface mode observed at about 60 meV in electron-energy-loss measurements at the Γ¯ point.