We calculate the transport critical-current density in a granular superconductor in magnetic fileds below about 5 x 10^-3 T. The field dependence in this region is assumed to be controlled by intragranular or intergranular Josephson junctions. Various model calculations are fitted to transport critical-current data on bulk Y1Ba2Cu3O_7-delta ceramic superconductors, whose average grain size somewhat exceeds 10 micro meters. The results yield an average junction cross-sectional area (thickness x length) of 4-6 micro m^2. If the junction are at the grain boundaries, a London penetration depth of about 150-300 micro m is inferred, consistent with other estimates. We conclude that Josephson junctions are limiting the transport critical current in these samples and that they lie at the grain boundaries. The parameters of the fit are not consistent with Josephson junctions at twinning boundaries.
Citation: Physical Review B
Pub Type: Journals