Effect of intergrain junctions and flux pinning on transport critical currents in YBa2Cu3O7−δ granular superconductors

Abstract

Studies of the dependence of the intergrain critical currents on temperature (60–90 K) and magnetic field (0–100 G) were performed for samples of YBa2Cu3O7−δ ceramics (YBCO) and of a 2-wt.% Ag-doped YBa2Cu3O7−δ composite (YBCO-Ag). The measurements were done using ceramic rings in a persistent mode and a scanning cryogenic Hall probe. This technique allowed us to satisfy conditions not available in the studies of critical currents before. A self-sustaining supercurrent (sensitive to any resistive dissipation) was used to detect the intergrain critical current Ic. Its magnitude and relaxation were measured using a contactless method (a Hall probe). The results of the measurements of Ic on temperature close to the intergrain Tc* confirmed the presence of the superconductor-insulator-superconductor tunnel intergrain junctions in YBCO and the superconductor–normal-metal–superconductor proximity junctions in YBCO-Ag. For YBCO, the Ic(T)=const(1-T/Tc*) dependence is not affected by the applied magnetic field, however, for YBCO-Ag the Ic(T)=const(1-T/Tc*)2 dependence is not preserved, suggesting a strong effect of magnetic flux creep on critical currents. The intergrain Tc* is about 5 K lower than the intragrain Tc and about 4 K lower than the ‘‘zero-resistance Tc’’ measured using a conventional I-V technique. Dissipation of a persistent current measured in the YBCO-Ag ring provided strong evidence that the transport current is controlled by an intergrain flux creep with the energy barrier proportional to Ic. The results revealed that the intergrain critical current density JcT in YBCO-Ag has lower values than JcT of YBCO, and the observed higher total critical currents are due to a larger surface area of grain-boundary conduction in this composite.

Source-reported materials — not catalogue approval

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