Transport and intragrain shielding currents in YBa2Cu3O7 single crystals

This paper reports on intragrain critical current densities of polycrystalline bulk samples and YBa2Cu3O7 single crystals (grown in Al2O3 crucibles) investigated by inductive flux profile measurements. In addition to the well known granularity in a polycrystal, granularity within these single crystals was detected. In analogy to polycrystalline samples increasing values of jc where observed with decreasing size of the powder particles ground from single crystal, thus giving strong evidence for the existence of intragrain junctions. In both ceramics and single crystals the investigation of the field dependence of jc reveals as a common feature a maximum at fields far below the upper critical field. This observation and the behavior of jc in the field region below this maximum are not in accordance with a critical current at expected from a pinning interaction but rather reveal junction character