Improved critical current characteristics in 1-2-3 oxide superconductors: weak flux pinning and percolative aspects

The authors have fabricated bulk samples of high-T c oxide superconductors with narrow inductive T c transitions ( approximately 3 K) and low resistivities ( rho (100 K):250-1000 mu Omega cm). They find such samples to have improved transport critical current density (J ct ) properties. Although J ct decreases rapidly with increasing field for fields <1 T, J ct is then approximately independent of field over the range 2-15 T at 4.2 K; at 77 K the field dependence is also much reduced. J ct is about the same whether the applied current is parallel to or perpendicular to the applied field. The high-field values of J ct are, however, still small (<or approximately=50 A cm -2 ), but are much higher than previous samples in which S-N-S tunnelling controlled J ct , reducing it to less than 1 A cm -2 in fields of approximately 1 mT at 77 K. DC magnetisation data demonstrate that the local critical current density within the individual grains is as high as 10 7 A cm -2 in fields up to 20 T at 4.2 K. The evidence that the low J ct is the result of easy flux shear along the grain boundaries, as well as that suggesting percolative processes operate, is discussed.