Tunneling in superimposed films

Tunneling experiments were performed on superimposed films of Pb-Al, Pb-Cd and Pb-Pt deposited and kept at 77^∘K until measured. The gap in the density of states induced by a 9000.25em0ex thick lead film in the Al, Cd and Pt films was measured as a function of their thickness. The experiments are in fairly good agreement with McMillan's theory, which was derived for pure films and scattering at the normal-superconducting interface. On the other hand, gapless superconductivity has been observed in lead films in contact with nickel and platinum films. The degree of gapless superconductivity (i.e. the density of states in the lead gap) has been measured from $dV/dI$ versus V curves as a function of the lead film thickness. As expected, a magnetic metal such as nickel has a larger effect on the density of states than a normal metal such as platinum, even if the effect is measured in sandwiches with the same transition temperature. The degree of gapless superconductivity in any of the sandwiches can be increased by applying a magnetic field; the relative conductance ρₛ/ρₙ (which is a neasure of the degree of gapless superconductivity) increases linearly with magnetic field close to the upper critical field as predicted theoretically by Maki.