Critical currents in polycrystalline high Tc superconductors

In this work we studied critical current measurements in magnetic fields up to 8T at 77 degree K of bulk and oriented thin films of YBa2Cu3O7δ. For bulk samples we found that the critical current was drastically reduced by two to four orders of magnitude at fields up to 2T. At these fields the critical current obeys a power law with an exponent n that correlates with the samples microstructure and average grain size. This power law behavior is discussed in terms of flux creep and Josephson junctions. A hysteretic behavior of the critical current between 103T to T was observed and found to be related to microstructure. l-V characterization of bulk and oriented samples was measured at temperature of 77 degree K and above in the presence of magnetic fields of varying strengths. For bulk samples, at 77 degree K, we found that the l-V characterization indicated that the critical currents were limited by flux creep and Josephson junctions corroborating our previous direct critical current measurements. For temperatures close to Tc, a flux flow behavior was found for bulk and oriented film. The upper critical field, Hc2, was measured using the Y. B. Kim formula. The existence of a vortex glass phase to vortex liquid phase transition was discussed. Our analysis of the l-V data did not confirm the existence of a phase transition. (author) 99 refs