A Helium-Atom-Scattering Study of the Structure and Phonons of the NiO(100)and CoO(100) Surfaces

The structure and dynamics of the NiO(100) and CoO(100) surfaces have been studied by high-resolution He atom scattering. Both crystals were cleaved in UHV providing unreconstructed surfaces of high quality. For both materials the surface phonon dispersion curves were measured over the entire Brillouin zone along the high-symmetry azimuthal directions. In addition to the Rayleigh mode, the Lucas mode (S4) and a longitudinal gap mode (S6) have been observed. Although both materials are very similar they reveal significantly different surface phonon spectra. A detailed lattice dynamical analysis indicates a charge redistribution at the surface that is presumably driven by the lower coordination of the outermost atoms.