An investigation of heating effects in the differential resistance of metal-superconductor junctions

A systematic study of heating and high voltage structure in normal metal/superconductor junctions was carried out using a new AC technique developed for the measurement of I-V and dV/dI-V characteristics of low resistance junctions. This technique uses a pulsed voltage ramp to reduce resistive heating in the junction by decreasing the amount of time current flows in the junction. This technique was used to investigate the differential resistance and the effect of heating in distributed point contact junctions between a tinned copper wire and two thin film metal-superconductor bilayers, Ag/Pb, and Ag/Tl{dollar}sb2{dollar}Ba{dollar}sb2{dollar}Ca{dollar}sb2{dollar}Cu{dollar}sb3{dollar}O{dollar}sb{lcub}10{rcub}{dollar} (Ag/Tl 2223). While heating was still observed in the Ag/Pb system, in the high-T{dollar}sb{lcub}rm c{rcub}{dollar} superconductor junction, Ag/Tl 2223 system no significant heating was seen. The differential resistance peaks in the Ag/Tl 2223 system are quite sensitive to temperature and therefore the location of these peaks acts as a thermometer for the junction. Since we observed no change in the location of these resonance peaks as the time the current flowed was varied, the temperature near the junction was shown to be constant to within 0.25 K, even using a DC voltage ramp. No high voltage structure or curvature at high voltages was seen in these junctions above the resonance structure up to the maximum voltage achieved with the Ag/Tl 2223 system, approximately 450 mV. The resonance structure in the differential resistance is therefore due to the presence of the high temperature superconductor and not to heating effects.