Quantum Size Effect In Superconducting Pb Islands

The lecture will be given by Prof. Wolf Dieter Schneider from the Swiss Federal Institute (EPFL) in Lausanne.
Abstract:
Using low temperature scanning tunneling spectroscopy at 5 and 50 K, we studied the linewidth of unoccupied quantum well states in ultrathin Pb islands, grown on Si(111) on two different Pb/Si interfaces. A quantitative analysis of the differential conductance spectra allowed us to determine the electron electron (e e), electron phonon (e ph), and the interface contributions to the lifetime. Layer dependent ab initio calculations of the e ph linewidth contribution are in excellent agreement with the data. Importantly, the sum of the calculated e e and e ph lifetime broadening follows the experimentally observed quadratic energy dependence (1).
The energy gap of these superconducting Pb islands, in a thickness range between 60 and 5 monolayer, was found to decrease from its bulk value as a function of inverse island thickness. Corresponding values of the critical temperature Tc, estimated using the bulk gap to Tc ratio, are in quantitative agreement with ex situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer dependent ab initio density functional calculations for freestanding Pb films show that the e ph coupling constant, determining Tc, decreases with diminishing film thickness (2). (1) I. P. Hong, C. Brun, F. Patthey, I. Yu. Sklyadneva, X. Zubizarreta, R. Heid, V. M. Silkin, P. M. Echenique, K. P. Bohnen, E. V. Chulkov, and W. D. Schneider, Phys. Rev. B (RC) 80, 081409 (2009).
(2) C. Brun, I. P. Hong, F. Patthey, I. Yu. Sklyadneva, R. Heid, P. M. Echenique, K. P. Bohnen, E. V. Chulkov, and W. D. Schneider, Phys. Rev. Lett. 102, 207002 (2009).