Surface lattice vibration and electron-phonon interaction in Pb(111) ultrathin superconducting films from first principles: Both with and without Si substrate

Abstract

The lattice vibration and electron-phonon (EP) coupling of three types of Pb ultrathin films were calculated from first principles. For free-standing films, our calculations showed that softening and stiffening of the surface phonons coexist. The obtained EP coupling λ and the superconducting transition temperature TC are smaller than their bulk values. For (1×1)-Pb/Si(111) films, the obtained λ and TC values in hexagonally close-packed (hcp) stacking are larger than those in face-centered-cubic (fcc) stacking. The largest TC value is obtained for N = 4ML (monolayers) in hcp stacking, which is very much consistent with the experimental study. Hence, our calculations imply that epitaxial ultrathin Pb films on the surface of Si(111) may possess an hcp rather tphan an fcc structure. Finally, the surface reconstruction and EP coupling of the √3× p √3 striped incommensurate phase with 4/3ML Pb on Si were studied, and the estimated TC in this phase is very close to its experimental value. Our calculations show clearly that the dynamical properties and EP coupling of ultrathin films depend strongly on the interlayer relaxation and the bonding environment between overlayer and substrate atoms at the interface. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.© IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.