Surface-phonon dispersion of a NiO(100) thin film

Abstract

A well-ordered 25 ML epitaxial NiO(100) film on Ag(100) as prepared by layer-by-layer growth has been characterized by high-resolution electron energy loss spectroscopy. Six different phonon branches have been identified in the $\overline{\ensuremath{\Gamma}}\phantom{\rule{0.16em}{0ex}}\overline{\mathrm{X}}$ direction of the surface Brillouin zone and are compared with first-principles phonon calculations. Whereas the surface Rayleigh mode shows a strong upward dispersion of 173 cm${}^{\ensuremath{-}1}$ in agreement with observations for the NiO(100) single crystal, the other surface phonons and surface resonances show only smaller dispersion widths in $\overline{\ensuremath{\Gamma}}\phantom{\rule{0.16em}{0ex}}\overline{\mathrm{X}}$ direction. The Wallis and the Lucas phonons are localized at 425 and 367 cm${}^{\ensuremath{-}1}$ at the $\overline{\ensuremath{\Gamma}}$ point, respectively. Additionally, two phonons are identified that have stronger weight at the zone boundary at 194 and 285 cm${}^{\ensuremath{-}1}$ and that become surface resonances at the zone center. The dominant spectral feature is the Fuchs-Kliewer (FK) phonon polariton at 559 cm${}^{\ensuremath{-}1}$, which is excited by dipole scattering and exhibits a rather broad non-Lorentzian lineshape. The lineshape is explained by a FK splitting resulting from the splitting of bulk optical phonons due to antiferromagnetic order. This view is supported by calculations of the surface-loss function from bulk reflectivity data.