Niobium trilayer Josephson tunnel junctions with ultrahigh critical current densities

We describe the fabrication and properties of Nb-AlOx-Nb Josephson tunnel junctions having critical current densities as high as 400 kA/cm2, roughly an order of magnitude larger than any previously reported for this materials system. The quality of the junction characteristics, as indicated by the current level at subgap voltages, is considerably lower in high-Jc than in low-Jc junctions. However, over the entire high-Jc range of 20-400 kA/cm2, the quality of our junctions remains the same (i.e., subgap current scales with critical current).