EVIDENCE FOR PARALLEL JUNCTIONS WITHIN HIGH-T(C) GRAIN-BOUNDARY JUNCTIONS

Half-integral constant voltage steps were observed in many high-T(c) grain-boundary Josephson junctions of YBa2Cu3O7-delta when a microwave field was applied. Five distinct observed behaviors of the widths of both integral and half-integral steps as a function of microwave amplitude, DELTAI(dc)(I(ac)), are reproduced by simulations of two or three junctions in parallel. This provides quantitative evidence that a single high-T(c) grain-boundary junction is composed of several junctions in parallel. These junctions are formed by the overlap of superconducting filaments on either side of the grain boundary, and the spacing between ones with relatively large critical currents is approximately 20 mum.