Enhancement of superconductivity far above the critical temperature in double-barrier tunnel junctions

Motivated by the observation of a superconducting energy gap far above the equilibrium critical temperature Tc in an Al film forming the center electrode of a Nb/AlOₓ/Al/AlOₓ/Nb structure we analyze the mechanism of gap enhancement in symmetric double-barrier superconducting tunnel junctions. It is found that such structures are very effective in creating a nonthermal distribution of quasiparticles in the middle electrode. At certain bias conditions this leads, according to the BCS gap equation, to the appearance of a nonzero superconducting energy gap even at temperatures up to several times the equilibrium Tc. So the double-barrier arrangement offers the remarkable possibility of making a material become superconducting by applying a voltage or passing a current. Calculated current-voltage characteristics exhibit current steps at voltages eV=2(ΔNb-ΔAl) and eV=2(ΔNb+ΔAl) in agreement with measured curves. Calculations of the thermodynamic stability of the nonequilibrium superconducting state indicate the possibility of hysteresis effects around these current steps.