Lattice dynamics of Cu(110): High-resolution He-scattering study

We report a high-resolution He-scattering study of the clean Cu(110) surface. We have determined the dispersion of the surface phonons along the Γ¯ X¯ and Γ¯ Y¯ symmetry directions. The 19.4-meV resonance near the Γ¯ point reported in previous electron-energy-loss spectroscopy studies has now also been observed using inelastic He scattering. Along the Γ¯ Y¯ direction we have obtained experimental evidence for phonon scattering from the transverse Cu bulk-band edge. Based on a dynamical model recently proposed by Lehwald et al., a lattice-dynamical slab calculation was performed; taking into account the experimentally determined lattice relaxation of Cu(110) and including surface stress within the topmost layer, good overall agreement is obtained with the experimental results. An examination of the calculated spectral density shows that the transverse bulk-band edge along the Γ¯ Y¯ direction provides a large vibrational amplitude at the surface, which is polarized in the sagittal plane and thus can be observed experimentally. The crossover between the bulk-band edge and a surface resonance of the same symmetry is avoided, and a hybridization of these modes is observed at a wave vector Q=0.70$Q_{\mathrm{BZ}(\mathrm{Y}\mathrm{¯}}$). Beyond this point the surface vibrational amplitude of the bulk-band edge strongly decreases, while the lower branch, which has then become a true surface mode, still has a large vibrational amplitude in the sagittal plane.