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Highlights • Decay properties of quantum-well states of a Pb overlayer on Si(111) are calculated. • Phonon-induced linewidths depend sensitively on binding energy. • Unoccupied Pb-derived states in the Si bulk band gap exhibit moderate linewidths. • Linewidths of occupied Pb states are enhanced by additional decay into Si bands. Graphical abstract Abstract We report a study of the electron–phonon contribution to the linewidth of quantum-well states in a surface superstructure formed by a 4/3-monolayer of Pb on Si(111), a dense phase with a 3 × 3 unit cell. Ab initio calculations based on the density-functional theory were carried out using a linear response approach in the mixed-basis pseudopotential representation. The phonon-induced contribution to the lifetime broadening is analyzed for both excited electrons and holes. The phonon contribution is found to be generally very sensitive to the energy position of the excited electron (hole) except for unoccupied Pb electronic states inside the Si band gap where the phonon-induced linewidth varies smoothly around ∼ 13 meV irrespective of electron energy. This differs for occupied Pb electronic states, which exhibit larger linewidths, or shorter lifetimes, because numerous phonon-mediated transitions to Si electronic bands substantially increase the electron–phonon coupling. [ABSTRACT FROM AUTHOR]

Highlights • Surface phonon dispersion curves of ultrathin 3–6 ML Pb films on Si(111) and Ge(111) measured with Helium atom scattering (HAS). • Density functional theory accounts for shallow Kohn anomalies observed in surface acoustic phonon branches. • The electron-phonon mass enhancement factor of ultrathin Pb films is measured from specular HAS intensity during growth. Abstract 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. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]