Phonons of fcc (100), (110), and (111) Surfaces Using Lennard-Jones Potentials.2. Temperature Dependence of Surface Phonons Studied with Molecular Dynamics

摘要

In this paper we present temperature dependent studies of the surface phonondispersion relations for fcc (100), (110), and 111) faces using molecular dynamics (MD) simulations and Lennard-Jones potentials. This study was conducted in order to investigate how anharmonic potential terms influence the dynamical properties of the surface. This was accomplished by examining the temperature dependence of the Q-resolved phonon spectral density function. All phonon frequencies were found to decrease linearly in T as the temperature was increased, while at low temperatures the phonon linewidths increased linearly with T. At higher temperatures, some of the phonon linewidths changed from having a linear to a quadratic dependence on T. The temperature at which this T to T change occurs is surface dependent and occurs at the lowest temperature on the (110) surface. The T 2 dependence arises from the increasing importance of higher-order phonon-phonon scattering terms. The phonons which exhibit T 2 dependence tend to be modes which propagate perpendicularly or nearly perpendicularly to the direction of maximum root-mean-squared displacement (RMSD). This is especially true for the linewidth of the S , mode at X on the (110) surface where, at T = 15 23% of the melting temperature, the RMSD perpendicular to the atomic rows become larger than the RMSD normal to the surface. Our results indicate that the dynamics on the (110) surface may be significantly influenced by anharmonic potential terms at temperatures as low as 15 % of the melting temperature.