Tunneling into Weakly Coupled Films of Aluminum and Tin in Proximity

Tunneling experiments have been performed into the N side of N-S (aluminum-tin) proximity sandwiches evaporated at room temperature onto an oxidized aluminum electrode B. The coupling between the N and S films was made weak by allowing slight oxidation to occur at the interface. When B is normal, the normalized tunneling conductance of these junctions in the vicinity of the critical temperature of the proximity sandwich is markedly different from that of junctions formed between B and an ordinary (BCS) superconductor. When B is superconducting and the thickness of the N film is made about dN100 1/10dS, where dS is the thickness of the S film, a double-peaked structure is observed in the tunnel conductance as a function of applied voltage. The properties of the proximity sandwich depend on the amount of oxidation at the N-S interface. Self-consistent calculations have been performed using the McMillan model of proximity sandwiches and treating the barrier transmission as a parameter. Comparison of these calculations with the experimental results shows satisfactory quantitative agreement.