Dispersion relations of surface phonons in LiF(001) and NaF(001)

Recent atom-scattering experimental data have stimulated a new investigation on the surface phonons of some alkali halides. In the framework of Green's-function method applied to breathing-shell model dynamics we have calculated the dispersion curves and a few $K$-selected densities of states of surface phonons for the (001) surface of NaF and LiF at room temperature. Particular attention is paid to the change in ion polarizabilities occurring at the surface and its effect on the Rayleigh wave frequencies at the zone boundary.