Cooling, conductance and thermometric performance of non-ideal normal metal-superconductor tunnel junction pairs

We have investigated the effect of a difference in the tunnelling resistances of the individual normal metal-insulator-superconductor (NIS) tunnel junctions in a double junction SINIS device, with particular emphasis on the impact on the conductance, cooling and thermometric performance. By solving the electrical and thermal equations of the junctions in a self-consistent way, we find that asymmetry gives rise to many new features, such as appearance of an excess sub-gap current, improved cooling performance, exhibition of negative differential resistance, and improved temperature range of thermometric sensitivity. Experiments were also carried out to complement some of the numerical results. In addition, we studied theoretically and experimentally the effect of a finite series resistance, which also causes an excess current in the subgap region, and a suppression of the conductance maxima at the gap edge. Experimental results agree well with the theoretical predictions.