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

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 $\overline{\Gamma }\overline{\mathrm{X}}$ 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${}^{-1}$ in agreement with observations for the NiO(100) single crystal, the other surface phonons and surface resonances show only smaller dispersion widths in $\overline{\Gamma }\overline{\mathrm{X}}$ direction. The Wallis and the Lucas phonons are localized at 425 and 367 cm${}^{-1}$ at the $\overline{\Gamma }$ point, respectively. Additionally, two phonons are identified that have stronger weight at the zone boundary at 194 and 285 cm${}^{-1}$ and that become surface resonances at the zone center. The dominant spectral feature is the Fuchs-Kliewer (FK) phonon polariton at 559 cm${}^{-1}$, 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.

Figure 1

Elastic-peak intensity as function of momentum transfer $\Delta k_{\parallel }$ along the $\overline{\Gamma }\overline{\mathrm{X}}\overline{{\Gamma }^{\prime }}$ direction. The energy of the scattered electron is 144 eV. The LEED pattern of NiO(100) for an electron energy of 149 eV is shown in the inset.

Figure 2

High-resolution electron energy loss spectrum for a 25-ML-thick NiO(100) layer on Ag(100) in specular scattering geometry for an electron energy of 4 eV.

Figure 3

HREEL spectra of the FK phonon region for a 25-ML-thick NiO(100) layer on Ag(100) for five different primary electron energies as indicated. The data by Oshima (Ref. 10) for a NiO(100) single crystal and a primary energy of 41 eV are shown as open squares for comparison.

Figure 4

Off-specular HREEL spectra for a 25 ML NiO(100) layer on Ag(100) measured with an electron energy of 81 eV along the $\overline{\Gamma }\overline{\mathrm{X}}$ direction. From bottom to top the off-specular angle is increased successively from 0 (bottom spectrum) by 3° for each spectrum. The corresponding momentum transfer (in Å${}^{-1}$) is labeled on the right side of the figure for every second spectrum. The structure of the first Brillouin zone in reciprocal space is shown in the inset.

Figure 5

Off-specular HREEL spectra at constant momentum transfer $\Delta k_{\parallel }$ = 0.76 Å${}^{-1}$ but for different electron energies (indicated in the figure). The contribution of the different phonon modes and their sum are shown with blue and red curves, respectively. In the bottom spectrum (a) the gain- and loss-RW peaks around the FK phonon peak are colored in green.

Figure 6

Off-specular HREEL spectra at the $\overline{\mathrm{X}}$ point measured with different electron energies as indicated. The contributions of the different phonon modes obtaining from the fitting procedure and their sum are shown with blue and red curves, respectively.

Figure 7

Experimental phonon dispersion (solid and open symbols) for the NiO(100) layer on Ag(100) along $\overline{\Gamma }\overline{\mathrm{X}}$ direction at 300 K. Solid circles denote the present HREELS data whereas the open squares and circles mark previous HREELS (Ref. 10) and HAS (Ref. 12) results for a NiO(100) single crystal, respectively. The solid circles of a given color indicate one phonon branch. The hatched areas mark the surface-projected bulk phonon bands as derived from our DFT+U frozen phonon calculations.

Figure 8

Relative intensity of the FK phonon as function of the electron kinetic energy in specular scattering geometry (a) and (b) as function of off-specular scattering angle for a fixed kinetic energy of 49 eV.

Figure 9

(a) Experimental NiO bulk-reflectivity data as adopted from Ref. 7 (solid symbols) and calculated reflectivity based on a generalized oscillator model with two (red solid line) and one oscillator (blue dashed line). (b) HREELS data (open circles) in the region of the FK phonon and calculated surface-loss function as derived from the bulk modeling in (a) (solid red and blue dashed lines). Additionally the surface-loss function with a reduced damping (see text) is displayed as green dotted line.