Phonons and electron-phonon anomalies in ultra-thin Pb films on Si(111) and Ge(111)

The surface phonon dispersion curves of ultrathin (3–6 ML) Pb layers grown on Si(111) and Ge(111) substrates, measured with inelastic He atom scattering, reveal shallow Kohn anomalies in the surface acoustic branches. Ab-initio calculations based on density functional perturbation theory for the 3–6 monolayers of Pb on a rigid substrate, besides reproducing well the observed dispersion curves, allow for the assignment of the anomalies to well defined flat segments of the Fermi contours. Unlike Pb films grown on Cu(111), where both the optical surface and the interface phonon branches above the bulk frequency maximum are observed, for Si(111) and Ge (111) only the surface-localized branch is observed, indicating the comparatively weaker film-substrate interaction. Consistently with the observation of superconductivity in ultrathin Pb films on Si(111) down to one monolayer (wetting layer), the mass-enhancement factor derived from the specular scattering intensity dependence on the film thickness is found for both substrates in the range of unity, however with slightly larger values for the Ge(111) substrate.