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 temperature dependencies of the mean-square displacements (MSD), the layer-by-layer stress tensors, and the surface phonon spectral densities were measured. A more pronounced increase in the MSD perpendicular to the atomic rows was observed as the temperature was increased as compared 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 first. Our calculations reveal that the surface phonon frequencies all decrease linearly with increasing temperature. Moreover, the surface phonon linewidths increase linearly with T at low T, and then exhibit an increased sensitivity to temperature variation, changing from a T to T2 dependence, approximately 150-degrees before the onset of defect creation at the surface. These simulation results imply that the Ni(110) and Cu(110) surfaces do not extensively roughen before the onset of adatom-defect formation, and, in confirmation of experimental findings, that the rapid decrease of specular intensity for helium or electron scattering at elevated temperatures is due to the influence of anharmonicity in the surface potential.