Thermal expansion and mean-square displacements of the Al(110) surface studied with medium-energy ion scattering

We report determinations of surface interlayer spacings and vibrational amplitudes of Al(110) in the temperature range 300-660 K, using medium-energy ion scattering with shadowing and blocking. We show that the surface vibrational amplitudes are anisotropic, and present results for their temperature dependence. The in-plane anisotropy changes sign between room temperature and 600 K. We also find evidence for a large mean-square displacement perpendicular to the surface for atoms in the second layer; a result that is quite nonintuitive but has been predicted theoretically. Such a finding also supports a predicted microscopic mechanism for surface premelting where adatoms are formed from displaced atoms in the second layer. Our data also show a smooth expansion of the first interlayer spacing in this temperature range.