High-T/sub c/ Josephson junctions by electron beam irradiation

Josephson junctions were produced by electron beam irradiation of patterned YBa/sub 2/Cu/sub 3/O/sub 7- delta / thin film tracks on MgO substrates. Single weak links show Fraunhofer-type critical current behavior as a function of field (I/sub c/(H)), indicating reasonable current uniformity across the junction. Simple low inductance superconducting quantum interference devices (SQUIDs) have also been made with greater than 70% critical current modulation at 25 K. The relatively high critical current density of these weak links means that it is possible to observe a crossover from small to large junction behavior at a temperature only a few degrees below the T/sub c/ of the junction. In the small junction regime the current-voltage (I-V) curve can be fitted by the resistively shunted junction (RSJ) model, whereas in the large junction regime the I-V curves change to be non-RSJ-like. The I/sub c/R/sub n/ (critical-current normal-resistance) product is typically around 0.05-0.2 mV at temperatures a few degrees below the T/sub c/ of the junction and, for a junction with high T/sub c/, can reach 1 mV at 4.2 K.<>