High critical current density obtained by focused ion beam patterning of high temperature superconducting thin films

In an attempt to pattern microbridges in a superconducting line by focused ion beam milling, we have obtained a very high critical current density: 6 × 108 A/cm2 at 77 K. Direct focused ion milling leads to microbridge structures as narrow as 200 nm. Ultrahigh current densities have already been reported by H. Jiang [Phys. Rev. Lett. 66, 1785 (1991)] in such ‘‘nanobridges.’’ We have also measured size-dependent critical current densities. We take into account the electrical field criterion and give a very simple interpretation of our experimental results based on the possibility of strong pinning in a very narrow bridge and the inhomogeneity of our films at a submicronic scale.