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 odd-symmetry shear horizontal surface modes. The scattering studies were carried out using a new multichannel detection HREELS spectrometer that achieves 3-4 meV energy resolution. The unique mechanical flexibility of the instrument offers out-of-plane scattering configurations in which odd-symmetry 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 even-symmetry phonon data (vibrations polarized in the sagittal plane) predict the odd-symmetry mode (normal polarization) to lie above the lowest energy even-symmetry 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 odd-symmetry surface phonon modes were detected. However along both directions, the odd-symmetry shear mode was the lowest energy mode. Measurements were taken on Ni(110) along |Gamma -|{X} and | Gamma-|{Y}, with out-of-plane studies completed along the | Gamma-|{X} direction only. Again, the odd-symmetry mode lies anomalously low at |{X}. The measurements of odd-symmetry shear modes on both surfaces depart significantly from predictions deduced on the basis of even-symmetry 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 even-symmetry 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