Surface-phonon dispersion curves of TiC(100)

The surface-phonon dispersion curves of the TiC(100) surface along both the Γ¯ M¯ and Γ¯ X¯ azimuths of the two-dimensional Brillouin zone have been measured by electron-energy-loss spectroscopy (EELS). We have observed two acoustical-phonon branches (Rayleigh and resonance modes) and three optical-phonon branches (Wallis, Lucas, and new modes). Those experimental curves are in good agreement with the theoretical ones calculated on the basis of a screened-shell model. In this calculation, the 20% stiffening of the stretching force constant between the first-layer Ti atom and the second-layer carbon produces a quasitransverse mode localized strongly at the second-layer carbon, which corresponds to the new observed surface mode. The local spectral densities calculated with use of these parameters have been compared with the experimental EELS spectra.