Localized surface optical phonons in a layered crystal: GaSe(001)

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Abstract
We report what we believe to be the first extensive experimental study of surface vibrations in a layered crystal by means of He inelastic scattering. In addition to the ordinary Rayleigh-wave branch, the dispersion curves for three strongly localized surface optical branches are unexpectedly found, in contrast with theoretical predictions for this class of materials. The various sequences of time-of-flight spectra leading to the dispersion curves and the peak amplitudes of the surface phonons are analyzed in detail. The nature of the surface dynamical perturbation required to reproduce the experimental dispersion curves is briefly discussed in the final section.