Persistent Superconductivity in Ultrathin Pb Films : A Scanning Tunneling Spectroscopy Study

By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18ML). The layer-dependent energy gap and transition temperature (T_c) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above Tc and the theoretical calculations, we found that the T_c oscillation correlates directly with the density of states oscillation at E_F. The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat. © 2006 The American Physical Society.