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 i n UHV providing unreconstructed surfaces of high quality. For both material s the surface phonon dispersion curves were measured over the entire Brillo uin zone along the high-symmetry azimuthal directions. In addition to the R ayleigh mode, the Lucas mode (S-4) and a longitudinal gap mode (S-6) have b een observed. Although both materials are very similar they reveal signific antly different surface phonon spectra. A detailed lattice dynamical analys is indicates a charge redistribution at the surface that is presumably driv en by the lower coordination of the outermost atoms. [S0163-1829(98)00743-7 ].