Supercurrent nonuniformities by ion implantation in niobium based Josephson junctions

Nitrogen ion implantation was used to induce local modifications of the superconductive properties of the niobium base layer in Nb-NbO _x-Pb Josephson tunnel junctions. A local depression of the niobium critical temperature and energy gap turns into a nonuniform critical current density profile J_c in the sample. Different profiles have been obtained by modifying the implanted region geometry and the nitrogen ion concentration. The external magnetic field dependence of the maximum DC Josephson current has been used as a diagnostic tool to obtain a preliminary characterization of the shape of J_c in a small sample. Preliminary investigations on long junctions are also reported