Geometric structure and surface vibrations of Cu(001) determined by medium-energy ion scattering

We have investigated the geometric structure and the vibrational properties of Cu(001) with medium-energy ion scattering. The surface structure follows the common trend for metal surfaces. The surface relaxation is found to be small and oscillatory with a top-layer contraction (d12) of -2.4% and a second-layer expansion (>d23) of +1%. In addition, we find that the surface vibrational amplitude is enhanced (as is usually the case) by 80%. A detailed analysis of our data shows an unexpected anisotropy of the vibrational amplitude, such that the out-of-plane vibrational amplitude is 30% smaller than the in-plane vibrational amplitude. This anisotropy can be removed by adsorbing 1/4 monolayer of sulfur on the surface. Our results are discussed in the context of surface stress and surface contractions.