Anomalies in the phonon dispersion of Mo(1 1 0)/Li—a Kohn anomaly or a stress induced effect?

The dispersion of surface phonons for the Li-covered Mo(1 1 0) surface displays significant deviations from the dispersion on the clean Mo surface near the N and H points of the surface Brillouin zone, at low and high coverages of Li, respectively. So far, these deviations have been attributed to Kohn anomalies, similar to those known for the hydrogen covered Mo-surface. The failure to find appropriate nesting vectors in the Fermi surface and the new measurements on the Li-induced surface stress cast doubt on this interpretation, however. In this paper we explore the possibility to describe the frequency shifts by normal lattice dynamics which models the surface stress by loaded springs between nearest and next nearest neighbors in the surface. We conclude that lattice dynamics can account for the observed “anomalies” and coupling to e–h pair excitations need not be invoked.