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 $d_N\simeq 100 \mathrm{\AA }\simeq \frac{1}{10}{d}_S$, where $d_S$ 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.