RECONSTRUCTION OF CURRENT FLOW AND IMAGING OF CURRENT-LIMITING DEFECTS IN POLYCRYSTALLINE SUPERCONDUCTING FILMS

Magneto-optical imaging was used to visualize the inhomogeneous penetr ation of magnetic flux into polycrystalline TlBa2Ca2Cu3Ox films with h igh critical current densities, to reconstruct the local two-dimension al supercurrent flow patterns and to correlate inhomogeneities in this flow with the local crystallographic misorientation. The films have a lmost perfect c-axis alignment and considerable local a- and b-axis te xture because the grains lend to form colonies with only slightly misa ligned a and b axes, Current flows freely over these low-angle grain b oundaries but is strongly reduced at intermittent colony boundaries of high misorientation, The local (<10-micrometer scale) critical curren t density J(c) varies widely, being up to 10 times as great as the tra nsport J(c) (scale of similar to 1 millimeter), which itself varies by a factor of about 5 in different sections of the film, The combined e xperiments show that the magnitude of the transport J(c) is largely de termined by a few high-angle boundaries.