Enhanced and correlated lattice vibrations of relaxed Cu(0 0 1) surface studied by high-resolution medium energy ion scattering

Abstract

The relaxed Cu(001) surface was analyzed by high-resolution medium energy ion scattering (MEIS) using 120keV He+ ions in a layer-by-layer fashion. We found the top-layer contraction of 1.6% and the 2nd-layer expansion of 0.9%, quite different from the close-packed (111) surface. Then enhanced thermal vibration amplitudes (TVAs) and the correlations between the 1st and 2nd nearest neighbor atoms in the [101]- and [001]-string were also determined simultaneously by MEIS spectrum analysis combined with Monte Carlo simulations of ion trajectories considering the surface relaxation and enhanced TVAs as well as correlations. We obtained the TVAs of the top-layer atoms strongly enhanced by 1.67±0.06 and 1.55±0.04 times that of the bulk (0.085Å) in the lateral and surface normal direction, respectively and the correlation coefficients of +0.33±0.04 and +0.28±0.04, respectively for Cu atoms in the [101]- and [001]-axis oscillating perpendicular to each string. The correlation between the 2nd nearest neighbor atoms in the [101]-string is small probably less than +0.2. The results obtained here are compared with the other experimental reports and the Debye approximation as well as molecular dynamics simulations using the embedded atom method.