CONDUCTANCE REGIMES IN SUPERCONDUCTING JUNCTIONS OF ATOMIC SIZE

A scanning tunneling microscope has been used to study the different c onductance regimes of superconducting Pb junctions of atomic size. Mea surement of the apparent tunneling barrier and imaging of probed areas before and after the spectroscopic measurement give information on th e surface condition and the geometry of the junction. The evolution of the differential conductance curves is followed from the tunneling re gime to contact as the junction is varied in a continuous and controll ed manner. The observed evolution of the conductance follows quantitat ively Octavio-Tinkham-Blonder-Klapwijk theory with the addition of a l ifetime broadening parameter, which accounts for the observed smearing of the superconducting gap features.