Extreme critical-temperature enhancement of Al by tunneling in Nb/AlOx/Al/AlOx/Nb tunnel junctions

High-conductance Nb/${\mathrm{AlO}}_{\mathit{x}}$/Al/${\mathrm{AlO}}_{\mathit{x}}$/Nb tunnel-junction devices with the Al thickness varying from 3 to 20 nm have been fabricated using a whole-wafer processing route. Current-voltage measurements show a step in the subgap structure corresponding to the difference in the energy gaps V=2(${\mathrm{\Delta }}_{\mathrm{Nb}}$-${\mathrm{\Delta }}_{\mathrm{Al}}$) of the two superconductors. The measured ${\mathrm{\Delta }}_{\mathrm{Al}}$ is large and persists to temperatures considerably in excess of the equilibrium critical temperature of the Al. This is interpreted as arising from a tunnel-induced gap enhancement which is an order of magnitude larger than has been seen previously.