G. Santoro

Theory of Brillouin scattering from surface acoustic phonons in supported films

The theory of the Brillouin scattering from the surface of a semi-infinite medium, developed previously, is extended to coated surfaces of cubic materials. The Brillouin cross section is obtained for any scattering geometry and for p and s polarisation of the incident and scattered light. The formulae for the cross section contain contributions from the ripple and the elasto-optic scattering mechanisms for the film as well as for the substrate. The theory is applied to a silica film deposited on crystalline silicon and the behaviour of the cross section against the film thickness is investigated. The authors find that there are strong interference effects among the various contributions to the cross-section, so that the intensity of the spectral lines dramatically oscillates by changing the thickness. Although in general the ripple scattering mechanism is the dominant one it is found that the elasto-optic coupling in the silica film is not negligible and sometimes is even prevailing. The discrete phonon spectrum (Rayleigh, Sezawa, Lamb modes) and the continuous spectrum (resonances, mixed modes) are both studied.

Surface phonon calculations for noble metals: Comparison with he-surface scattering experiments

Abstract We evaluate the surface phonon dispersion relations of Ag(111) and Au(111) within a force constants parametrization of the bulk dynamics. By changing the surface force constants we are able to explain all the salient features appearing in the experimental time of flight spectra of a 7.9 meV He beam. The parallel momentum Q has been considered along the [110] and [112] directions of the surface Brillouin ozne. We show that the peaks of the time of flight spectra are related to the Rayleigh wave, to the pseudo Rayleigh wave and to a new resonance of longitudinal character.

Surface resonant phonons of Ag(110)

Abstract A study of the dynamics of the Ag(110) surface in the ḡGȲ direction is presented. The experimental TOF spectra show the existence of two low frequency modes with high scattering intensity and a dispersionless high frequency mode with a smaller intensity. The theoretical analysis based on a force constant model that contains long-range central interactions and short-range angular forces explains accurately both the dispersions and the scattering intensities of the three modes in terms of resonant and localized vibrations related to the modifications of the surface force constants caused by the different role played by the many-body forces in the surface region.

Debye-Waller factor for He/Pt(111)

Abstract We report accurate measurements of the dependence of the specular reflectivity on surface temperature and on incident angle, for He scattering off Pt(111). The usual Debye-Waller formula with a Beeby correction fits the data, provided that the effective well depth is adjusted. We show that a full calculation of the Debye-Waller exponent also reproduces the data. The inelastic atom-surface interaction and the surface phonon spectrum used in the calculation are the ones that reproduce the time-of-flight data obtained in separate experiments. No further adjustable parameters are needed.

Electronic structure of nonpolar surfaces of II–VI compounds

The nature of surface states in cleaved surfaces of II–VI compounds with zincblende structure is studied. The tight‐binding method is used to calculate the electronic structure of a layer of twelve (110) planes. The results show various surface bands both in the gap and in empty lenses of the projected bulk band structure. The nature of these states is discussed and a comparison with the available experiments is presented. The results support the model of ionic surface states previously proposed for these surfaces.

Surface Brillouin scattering from acoustic phonons. II. Application to semiconductors

The theory of Brillouin scattering from corrugated surfaces developed previously has been applied to semiconductors. The available spectra measured with 5145 AA light has been used for evaluating the elasto-optic constants (k11, k12, k44) of the medium. For GaAs k11=42.5, k12=51 and k44=25.5. This is the first determination of these constants in GaAs at this wavelength, where other techniques are not applicable. For silicon k11=53, k12=25 and k44=23. It is also shown that in opaque semiconductors the elasto-optic coupling depends strongly on the surface orientation.

Theory of inelastic Ne scattering from the Ni(111) surface

The recent inelastic Ne scattering data for Ni(111) by Feuerbacher and Willis are theoretically analyzed in the framework of the distorted wave Born approximation. A Ne-surface soft potential model is inferred from two-body Ne-Ni interactions and the many-body effects due to the phonon-induced surface electron charge redistribution are qualitatively described by three-body forces. The surface-projected phonon densities for the Ni(111) surface are calculated from a force constant model including three-body forces. The cut-off produced by the softness of the Ne-Ni potential accounts for the observed cross-over from Rayleigh-wave to bulk phonon scattering regimes. The one-phonon picture correlates well with experiments for outgoing angles slightly apart from the specular condition (Rayleigh-wave regime) whereas for larger deviations from the specular condition (bulk phonon regime) multiphonon contributions are important.

Far-Infrared Vibrational Spectroscopy in CrSi2

We present and discuss reflectance and trasmittance measurements on CrSi2 polycrystalline films in the (100 ÷ 700) cm-1 wavenumber range. The spectra show that the compound has a nonmetallic behaviour. The strong and sharp structures observed are interpreted on the basis of the symmetry properties of the normal modes of CrSi2, and their strength is attributed to a dynamical charge. Moreover, preliminary lattice dynamics calculations allow to explain the relative intensity of the infrared-active modes.

Inelastic scattering of helium atoms from the phonons of the reconstructed Au(111) surface

We present inelastic atom-surface phonon scattering time-of-flight spectra for the reconstructed Au(111) surface. The shapes of the spectra are compared with calculated reflection coefficients based on a simplified lattice dynamical model for the reconstructed surface. The force constants were determined from a best fit of the surface phonon dispersion curves and the lateral atom-atom interactions in the surface layer were found to be 30% of the bulk value. The theoretical spectra reproduce the overall lineshape of the experimental spectra and reveal additional features which can be attributed to the reconstruction of the surface layer with respect to the substrate.

sp-d hybridization effects on the electronic structure of the (100) surface in copper

The electronic structure of the (100) surface of Cu has been calculated within a tight-binding description of the electron states, using the layer method. With respect to the calculation with pure d bands, it is found that the inclusion of sp-d hybridization modifies the d surface states and introduces a new state localized in the first two layers. This state is identified as a Schockley-type surface state arising at the lower edge of sp-d gap.