Shear horizontal phonons on Ni(110)

The dispersion curve of the shear horizontal surface phonon along \ensuremath{\Gamma}\ifmmode\bar\else\textasciimacron\fi{} X\ifmmode\bar\else\textasciimacron\fi{} ([1\ifmmode\bar\else\textasciimacron\fi{}10]) on Ni(110) has been measured with use of high-resolution electron-energy-loss spectroscopy. The frequency of this phonon at X\ifmmode\bar\else\textasciimacron\fi{} is found to be 169 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Theoretical calculations provide an excellent account of the loss spectra measured in scattering geometries where excitation of shear horizontal modes is forbidden, and those in which they contribute strongly. The frequency of the shear horizontal phonon at Y\ifmmode\bar\else\textasciimacron\fi{} is measured also, and found to be approximately 116 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. A model introduced earlier for Ni(110) is in reasonable accord with the data, though a full account requires the introduction of noncentral forces into the lattice dynamics.