Modification of current-voltage characteristics of double-barrier tunnel junctions under the influence of quasiparticle extraction

Abstract

Enhancement of the magnitude of the value corresponding to the superconducting energy gap of Al${}^{\ensuremath{'}}$ in Nb/Al-AlO${}_{x}$-Al${}^{\ensuremath{'}}$-AlO${}_{x}$-Al/Nb and Nb/Al/Nb/Al-AlO${}_{x}$-Al${}^{\ensuremath{'}}$-AlO${}_{x}$-Al/Nb/Al/Nb devices by tunnel extraction of quasiparticles has been investigated experimentally. The enhancement is observed both at the gap-difference and gap-sum voltage. In addition to the gap-difference and gap-sum features, there is a feature in the $I$-$V$ characteristic which corresponds to the sum of the energy gaps of the external electrodes. The dc and ac Josephson currents appear in the devices below the same critical temperature ${T}_{\mathrm{c}\mathrm{}\mathrm{eq}},$ which is found to be ${T}_{\mathrm{c}\mathrm{}\mathrm{eq}}=2.35$ and 2.60 K for the devices of the first and second types, respectively. Below ${T}_{\mathrm{c}\mathrm{}\mathrm{eq}},$ an enhancement of the dc component of ac Josephson current has been observed at $V>0$, as compared with the magnitude of the dc Josephson current at $V=0$.