TEMPERATURE-DEPENDENCE AND ANHARMONICITY OF PHONONS ON NI(110) AND CU(110) USING MOLECULAR-DYNAMICS SIMULATIONS

Molecular dynamics simulations were performed for Ni(110) and Cu(110) using Finnis-Sinclair model potential. During the simulations the temp erature dependencies of the mean-square displacements (MSD), the layer -by-layer stress tensors, and the surface phonon spectral densities we re measured. A more pronounced increase in the MSD perpendicular to th e atomic rows was observed as the temperature was increased as compare d to either the other in-plane direction or along the surface normal. Also, at each temperature studied, the MSD along the direction normal to the surface were always larger in the second layer than in the firs t. Our calculations reveal that the surface phonon frequencies all dec rease linearly with increasing temperature. Moreover, the surface phon on linewidths increase linearly with T at low T, and then exhibit an i ncreased sensitivity to temperature variation, changing from a T to T2 dependence, approximately 150-degrees before the onset of defect crea tion at the surface. These simulation results imply that the Ni(110) a nd Cu(110) surfaces do not extensively roughen before the onset of ada tom-defect formation, and, in confirmation of experimental findings, t hat the rapid decrease of specular intensity for helium or electron sc attering at elevated temperatures is due to the influence of anharmoni city in the surface potential.