Additional boundary conditions and surface exciton dispersion relations

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Abstract
The surface-exciton dispersion curves in ZnO are derived from the surface impedances developed by Fuchs and Kliewer (FK) and Rimbey and Mahan (RM) including retardation. There exists a distinctive splitting between the two dispersions, the FK additional boundary conditions having longitudinal character, the RM additional boundary conditions being transverse. Surface-mode attenuation due to spatial dispersion is more pronounced in the RM formalism, although inclusion of a phenomenological damping parameter does not alter either dispersion curve.