Anharmonicity on Al(100) and Al(111) surfaces

We present a molecular-dynamics analysis of the stable nonmelting (100) and (111) surfaces of Al. A many-body potential derived from first-principles calculations is used. The molecular-dynamics method includes anharmonic effects of all orders. We study static and dynamical properties of the surface. An expansion of the (111) surface and a contraction of the (100) surface results from the calculations. At low temperature, the vertical mean-square vibrational amplitude is larger than the in-plane component, while at higher temperature the in-plane component approaches the vertical one. Both components are at least twice as large as the bulk value. The interactions due to the surface decay very rapidly going into the crystal from the surface, as indicated by the analysis of the Debye-Waller factor. The evaluated linewidths for the Rayleigh surface phonon reproduce quite well the temperature dependence of the He-surface scattering data. The experimental behavior of the energy shifts, as a function of the scattering momentum transfer, presents a minimum inside the Brillouin zone, which is also found by our calculations. The surface energy shifts are about 30% larger than the bulk ones at the same temperature. The effect of surface anharmonicity is much larger for the static properties than for the dynamical properties.