Gapless Superconductivity Induced by the Proximity Effect

Fulde and Maki have calculated the tunneling conductance of a superconducting film in the gapless regime. We have studied the tunneling characteristics of $\mathrm{A}\mathrm{l}-{\mathrm{Al}}_2{\mathrm{O}}_3-\mathrm{P}\mathrm{b}-M$ junctions (where $M$ was respectively Ni, Fe, and Pt) as a function of temperature and of the lead film thickness. In the case of Pb-Ni and Pb-Fe sandwiches, the lead film is gapless and the ratio of the conductances at zero bias in the normal and superconducting states approaches unity linearly as the temperature approaches $T_c$ (the sandwich transition temperature). The slope of this linear portion is proportional to $C\left(t_c\right)$, which is a universal function of $t_c=\frac{T_c}{T_{\mathrm{cs}}}$, where $T_{\mathrm{cs}}$ is the transition temperature of pure lead. In agreement with theory $C\left(t_c\right)$ approaches 2 as $t_c$ approaches unity; but sandwiches with $t_c<1\mathrm{}$ are found to be more gapless than predicted by theory. The same conclusions apply to Pb-Pt sandwiches which exhibit quasigapless superconductivity. Finally, Pb-Pt and Pb-Ni sandwiches with the same $T_c$ display the same degree of gapless superconductivity. On the other hand, Josephson tunneling experiments were performed with Cr-Pb-PbO-Pb-Cr junctions. Although the magnitude of the Josephson current seems to decrease with increasing degree of gapless superconductivity, its temperature and field dependences are very similar to those previously observed in superconductors displaying an energy gap.