Reconstruction of current flow and imaging of current- limiting defects in polycrystalline superconducting films

Magneto-optical imaging was used to visualize the inhomogeneous penetration of magnetic flux into polycrystalline TIBa₂Ca₂Cu₃O(x) films with high critical current densities, to reconstruct the local two-dimensional supercurrent flow patterns and to correlate inhomogeneities in this flow with the local crystallographic misorientation. The films have almost perfect c- axis alignment and considerable local a- and b-axis texture because the grains tend to form colonies with only slightly misaligned a and b axes. Current flows freely over these low-angle grain boundaries but is strongly reduced at intermittent colony boundaries of high misorientation. The local (<10-micrometer scale) critical current density J(c) varies widely, being up to 10 times as great as the transport J(c) (scale of ~1 millimeter), which itself varies by a factor of about 5 in different sections of the film. The combined experiments show that the magnitude of the transport J(c) is largely determined by a few high-angle boundaries.