54.
Considering the (117) face of an fcc crystal, the dispersion relations of localized surface modes and resonances have been calculated using the method of generating coefficients of Green functions. The force between atoms are limited to the central force between the nearest neighbours. In the direction of propagation orthogonal to a step, for each wavevector value, a localized surface mode and two resonances have been found. Their polarization amplitude and width have been calculated. It is shown that these results can be qualitatively understood by folding three times the dispersion relation of the (001) face which is the crystallographic plane of the terrace. In the direction parallel to the step, the dispersion relations are obtained by the superposition of the sagittal and transverse dispersion relation of the (001) face. The (117) configuration introduces a coupling between these two polarizations and some resonances appear. A general rule which enables one to predict qualitatively the shape of the dispersion relation for any vicinal surface of type (11
m) is expressed.
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