HIGH-TC JOSEPHSON JUNCTIONS BY ELECTRON BEAM IRRADIATION

Josephson junctions are produced by electron beam irradiation of pattemed YBa2Cu3O7-delta thin film tracks on MgO substrates. Single weak links show Fraunhofer type critical current behaviour as a function of field (I-c( H)), indicating reasonable current uniformity across the junction. Simple low inductance SQUIDs have also been made with >70% critical current modulation at 25K. The relatively high critical current density of these weak links means that it is possible to observe a cross-over from small to large junction behaviour at a temperature only a few degrees below the T-c of the junction. In the small junction regime the current-voltage (IV) curves can be fitted by the resistively shunted junction (RSJ) model, whilst in the large junction regime the IV curves change to be non-RSJ like. The IcRn product is typically around 0.05 - 0.2 mV at temperatures a few degrees below the Tc of the junction and for a junction with a high T-c can reach 1mV at 4.2K. The width of the irradiated line is found to be in the range 20-100nm.