The Study of Shear Horizontal Phonons at SILVER(110) and NICKEL(110) Surfaces
Abstract
High resolution electron energy loss spectroscopy (HREELS) has been used to study the surface phonon modes on Ag(110) and Ni(110). The measurements include the detection of oddsymmetry shear horizontal surface modes. The scattering studies were carried out using a new multichannel detection HREELS spectrometer that achieves 34 meV energy resolution. The unique mechanical flexibility of the instrument offers outofplane scattering configurations in which oddsymmetry vibrational modes may be detected. Lattice dynamical analysis was conducted which explored the effects on the surface interaction model arising from the inclusion of the shear horizontal phonon data. Surface vibrational spectroscopy is a powerful technique for studying the structural and dynamical properties of surfaces. Surface phonon dispersion measured previously by inelastic electron and Helium atom scattering has been used in conjunction with lattice dynamical calculations to explore surface interaction potentials. The specific surface models for Ag(110) and Ni(110) based on previous evensymmetry phonon data (vibrations polarized in the sagittal plane) predict the oddsymmetry mode (normal polarization) to lie above the lowest energy evensymmetry mode along high symmetry directions in the surface Brillouin zone. Measurements were taken on Ag(110) along Gamma{X} and Gamma{Y}, and both even and oddsymmetry surface phonon modes were detected. However along both directions, the oddsymmetry shear mode was the lowest energy mode. Measurements were taken on Ni(110) along Gamma {X} and  Gamma{Y}, with outofplane studies completed along the  Gamma{X} direction only. Again, the oddsymmetry mode lies anomalously low at {X}. The measurements of oddsymmetry shear modes on both surfaces depart significantly from predictions deduced on the basis of evensymmetry data, and cannot be adequately accounted for by the previous surface models. A lattice dynamical model of Ag(110) was developed which accounts for both odd and evensymmetry data. It is found that the choice of force constants at the surface is greatly influenced by the shear horizontal modes, and hence these modes must be properly treated in the lattice dynamical analysis.
 Publication:

Ph.D. Thesis
 Pub Date:
 1992
 Bibcode:
 1992PhDT........73Y
 Keywords:

 SILVER(110) SURFACES;
 NICKEL(110) SURFACES;
 Physics: Condensed Matter