Characterization of surface phonons on Cu(001) and Ag(001): First-principles phonon calculations with experimental and theoretical studies of high-resolution electron-energy-loss spectra

A parameter-free first-principles phonon calculation is combined with a multiple-scattering electron-energy-loss-spectra calculation to characterize surface phonons of Cu(001) and Ag(001). The electron-energy-loss data on Cu(001) reveal the gap mode ${\mathit{S}}_6$ at X¯, which has been detected before only for Ni(001). The calculations also discover two additional surface modes on these surfaces, and the inelastic-electron-scattering cross sections of these modes are investigated. With the discovery of these modes, and knowledge about their inelastic-cross-section behavior, data of inelastic electron scattering are very satisfactorily explained. We also suggest that the mode found at X¯ could explain a recent He-atom scattering experiment. A comprehensive picture is established for the surface phonons on Cu(001) and Ag(001).