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

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Abstract
The authors have fabricated bulk samples of high-Tc oxide superconductors with narrow inductive Tc 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 (Jct) properties. Although Jct decreases rapidly with increasing field for fields 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. Jct is about the same whether the applied current is parallel to or perpendicular to the applied field. The high-field values of Jct are, however, still small (-2), but are much higher than previous samples in which S-N-S tunnelling controlled Jct, 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 107 A cm-2 in fields up to 20 T at 4.2 K. The evidence that the low Jct is the result of easy flux shear along the grain boundaries, as well as that suggesting percolative processes operate, is discussed.