Geometric Structure, Pseudo Band Gaps, and Surface Vibrational Resonances on Metal Surfaces

摘要

It is shown by surface lattice dynamics that a new class of surface vibrational resonances arises in those frequency regions where there is a strong depletion in the bulk phonon density of states. The presence of these pseudo band gaps is due to the higher Fourier components in the phonon dispersion relations introduced by the particular coordination of atoms in layers parallel to the surface. This phenomenon is illustrated for the fcc (110) surfaces of Cu and Ni and bcc(111) surface of Fe. The pseudo band gap is found to be more pronounced for the longitudinal phonons propagating in the bcc (111) than the fcc (110) directions due to the additional strong multiple interlayer forces arising from its geometric structure. A quantitative analysis based on surface lattice dynamics of the recorded electron energy loss spectra of Cu and Ni suggests that the surface interlayer force constant attains the same value as in bulk, and that the two outermost layers give the dominant contribution to the dipole activity. This resonance is found to exist throughout the gamma-X direction and makes an avoided crossing with a resonance derived from a band gap at the X-point. This novel dispersion behavior should be possible to observe by electron or atom scattering at larger parrallel wavevector transfers.