Surface phonons of NiO(001) ultrathin films grown pseudomorphically on Ag(001)

摘要

For a ultrathin NiO(001) film of 4 monolayers (ML) thickness grown onAg(001), the vibrational properties have been determined by high-resolutionelectron energy loss spectroscopy (HREELS). For the well-orderedpseudomorphically grown film, nine phonon modes have been identified and theirdispersions have been revealed along the Gamma-X high-symmetry direction. Thecomparison with phonon data for a 25 ML thick NiO(001) film shows that the NiO(001) phonon properties are already fully developed at 4 ML. Significantdifferences are found for the surface-localized phonon S6 which has anincreased dispersion for the ultrathin film. The dipole-active Fuchs-Kliewerphonon-polariton exhibits a narrower lineshape than the mode found for asingle-crystal surface, which might hint to a reduced antiferromagneticcoupling in the ultrathin film.