EXTREME CRITICAL-TEMPERATURE ENHANCEMENT OF AL BY TUNNELING IN NB/ALOX/AL/ALOX/NB TUNNEL-JUNCTIONS

High-conductance Nb/AIO(x)/Al/ALO(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(DELTA-Nb - DELTA-Al) of the two superconductors. The measured DELTA-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.