Surface structures and thermal vibrations of Ni and Cu thin films studied by extended x-ray-absorption fine structure

Temperature- and angle-dependent extended x-ray-absorption fine-structure spectra of Cu and Ni thin films grown epitaxially on highly oriented pyrolytic graphite were measured and analyzed in order to investigate the dynamical properties of the surface metal-metal bonds. From the mean-square relative displacements of the metal-metal bonds, the effective Debye temperatures of the surface in-plane and out-of-plane bonds were determined. The surface bonds were found to be much softer and anharmonic, especially for the surface normal direction, than those of the corresponding bulk metals. We have also carried out classical Monte Carlo simulations for Cu films based on the embedded-atom method. The experimental results were reproduced qualitatively well and it was clarified that the out-of-plane bonds between the first and the second layer are noticeably weaker than the other bonds such as the in-plane bond in the first layer.