Effect of intergrain junctions and flux pinning on transport critical currents in YBa2Cu3O7- delta 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 ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ ceramics (YBCO) and of a 2-wt.% Ag-doped ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ 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 ${\mathit{I}}_{\mathit{c}}$. Its magnitude and relaxation were measured using a contactless method (a Hall probe). The results of the measurements of ${\mathit{I}}_{\mathit{c}}$ on temperature close to the intergrain ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ 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 ${\mathit{I}}_{\mathit{c}}$(T)=const(1-T/${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$) dependence is not affected by the applied magnetic field, however, for YBCO-Ag the ${\mathit{I}}_{\mathit{c}}$(T)=const(1-T/${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$${)}^{2}$ dependence is not preserved, suggesting a strong effect of magnetic flux creep on critical currents. The intergrain ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$ is about 5 K lower than the intragrain ${\mathit{T}}_{\mathit{c}}$ and about 4 K lower than the ``zero-resistance ${\mathit{T}}_{\mathit{c}}$'' 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 ${\mathit{I}}_{\mathit{c}}$. The results revealed that the intergrain critical current density ${\mathit{J}}_{\mathit{c}\mathit{T}}$ in YBCO-Ag has lower values than ${\mathit{J}}_{\mathit{c}\mathit{T}}$ of YBCO, and the observed higher total critical currents are due to a larger surface area of grain-boundary conduction in this composite.