Electron-phonon coupling in quantum-well states of the Pb/Si(1 1 1) system

The electron-phonon coupling parameters in the vicinity of the Γ point, λ(Γ), for electronic quantum well states in epitaxial lead films on a Si(1 1 1) substrate are measured using 5, 7 and 12 ML films and femtosecond laser photoemission spectroscopy. The λ (Γ) values in the range of 0.6-0.9 were obtained by temperature-dependent line width analysis of occupied quantum well states and found to be considerably smaller than the momentum averaged electron-phonon coupling at the Fermi level of bulk lead, (λ = 1.1-1.7). The results are compared to density functional theory calculations of the lead films with and without interfacial stress. It is shown that the discrepancy can not be explained by means of confinement effects or simple structural modifications of the Pb films and, thus, is attributed to the influence of the substrate on the Pb electronic and vibrational structures.