The spatial variation of the current across the boundary in several YBa2Cu3O7−x grain boundary Josephson junctions was determined using direct methods. A phase retrieval algorithm was used to calculate the positional critical current density J(x) from critical current versus applied magnetic field, Ic(B), measurements. The current distributions were highly nonuniform along the length of the junctions. These measurements are consistent with existing filamentary grain boundary models, low temperature scanning microscopy studies, and laser scanning microscopy studies of high Tc grain boundaries. The very large scatter in the critical currents reported in the literature for grain boundaries of the same macroscopic geometry appear to be due to the underlying variations in local critical currents.

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