ANHARMONIC EFFECTS AT METAL-SURFACES

We present here results of temperature-dependent studies of the dynami cs of several fee metal surfaces using classical molecular dynamic sim ulations and interaction potentials from the embedded atom method. At room temperature, the mean square vibrational amplitudes of the (100) surface of Ag, Cu and Ni are found to be isotropic, while on (111) the out-of-plane vibration is larger and on the (110) surface the in-plan e motion along (001) is the dominant one. Beyond room temperature, anh armonic effects at the surface become noticeable and at a temperature somewhat larger than half the melting temperature the in-plane, anharm onic vibrational amplitudes on the (110) surface become so large that adatoms and vacancies are created. This leads the surface to disorder and eventually to premelt. On the (100) surface, however, adatoms and vacancies do not appear until close to the melting temperature, while Cu(110) is found to premelt at about 1200K and Ag(110) at about 1050K. The surface thermal expansion of Ag(110) is also found to be signific antly larger than for the bulk. The phonon frequency shifts and line-w idth broadening are calculated as a function of surface temperature. T hese characteristics on Ag(110) are compared with those on Cu(110).