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

摘要

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