Subgap Tunneling Current at Low Temperature in Nb/Al-AlN/Nb SIS Junctions

The quasiparticle tunneling current of Nb/Al-AlN/Nb SIS junctions are studied both experimentally and numerically by taking into account not only the quasiparticle states produced in the energy gap by the imaginary part of the gap energy but also a bound state near the Fermi level. It is shown that the calculated I-V curves agree very well with those of SIS junctions measured at temperatures below 4.2 K. It is found that the magnitude of the imaginary part of the gap energy reaches lower value at low temperature, which means that recombination time of the quasiparticles is saturated at low temperatures.