Experimental observation of an enhancement of superconducting gap in niobium thin film of superconducting double barrier structures.

An enhancement of the superconducting gap was observed experimentally in the thin interlayer Nb' film of superconducting double barriers structure Nb-Al2O3-Nb'-Al2O3-Nb. There were investigated the superconducting double barrier structures with 10 and 20 nm thickness of the interlayer Nb' film, where the injection rate of quasiparticles over the tunnel barrier was small (less than the reverse of the inelastic relaxation time of quasiparticles in the interlayer). The observed value of Nb' superconducting gap was increased up to 93% of the equilibrium one at T=0. There is a good agreement between the obtained experimental results and numerical calculations, made on the basis of the quasiparticle balance model, where we take into account the energy dependence of an inelastic relaxation time of quasiparticles with the energy, close to the superconducting gap.