Impact of non-idealities on the conductance characteristics of superconductor-insulator-normal metal-insulator-superconductor tunnel junctions

We have investigated the effect of asymmetry in tunnelling resistance of individual normal metal-insulator-superconductor (NIS) tunnel junctions that constitute a SINIS pair, both experimentally and theoretically. Ours results clearly demonstrate that any finite asymmetry in the tunnelling resistance gives rise to an excess current, as compared to its symmetric counterpart, both below and around the gap edge. The signature of this excess current is visible almost up to the critical temperature. We find that this apparent broadening of the density of states is purely electrical in origin. Our calculations also show that any finite resistance that is in series with the tunnelling resistance, such as the resistance of normal metal island or the line resistance in case of two probe measurements, leads to a suppression of the conductance maxima at the gap edge. This is a manifestation of the finite voltage drop across the series resistor. Our experimental results validate our theoretical prediction.