RESISTIVE TRANSITION FOR YBA2CU3O7-DELTA-Y2BACUO5 COMPOSITES - INFLUENCE OF A MAGNETIC-FIELD

The resistive transitions of YBa2Cu3O7-δ-Y2BaCuO5 granular composites (denoted as Y123/Y211) have been systematically investigated under applied magnetic fields up to 4 kG at a fixed low transport current density of 1.5 mA cm-2. In the region of the superconducting transition the samples showed two distinct sections, a steep part associated with the onset of superconductivity in the individual grains and a transition tail due to the weak links coupling the grains. For each composite the steep section remained unchanged with applied field while the tails moved considerably to lower temperatures with this dissipation region being very sensitive to both transport current and applied field as the percolation limit was approached. The resistivity tails have been interpreted using the Ambegaokar and Halperin (AH) phase-slip model and a four-variable fitting procedure. The AH parameter γ(H)T=0 has been used to estimate the critical current density, at zero temperature, in the grain boundaries for the different composites at various magnetic fields. The magnetic field dependence of γ(H) was found to be H-n with n=0.5 for pure Y123 and 0.3 for the other composites. The resistivity data near Tc (ρ=0) did not conform with the AH model and have been interpreted in terms of the thermally activated flux-creep model. Pinning energies have been determined and were also found to follow the applied magnetic field according to H-α with α=0.60 for all composites. © 1994 The American Physical Society.