Molecular dynamics study of the vibrational and transport properties of copper adatoms on the (111) copper surface; Comparison with the (001) face

The vibrational properties and the self-diffusion process of single adatoms on the Cu(lll) surface have been studied and compared with the corresponding ones of the Cu(001) face, using molecular dynamics simulation based on a n-body potential. From the calculated phonon spectral densities it comes out that the adatoms modify the surface phonon spectrum and introduce new vibrational modes. The frequencies of these new modes suggest, in the in-plane directions, looser coupling of the adatoms with the surface atoms of the (111) than those of the (001) face and stronger coupling in the perpendicular direction. This is in agreement with the temperature dependence of the relaxed positions of both surface atoms and adatoms. The effect is more pronounced for the (001) face, indicating that this surface presents anharmonic behavior at high temperatures. In addition, the mean-square-displacements of the surface atoms in the normal direction, are greater in the case of the (001) face. The vibrational amplitudes of the adatoms on the (111) surface are smaller than those of the surface atoms, for a wide temperature range. The adatom self-diffusion on the (111) surface is a thermally activated process, exhibiting Arrhenius behavior at two distinct temperature regions. The detailed analysis of the diffusion process revealed a new hopping mechanism.