Factors dominating the transport critical current behavior in high-T C superconductors

The previous experimental studies on transport Jc and grain boundary (micro)structure of YBa2Cu3Ox have been summarized. A novel interpretation is proposed for the transport current behavior in high-Tc superconductors. Based on extensive transport, magnetization, and electron microscopy experimental data reported previously, we show that the transport Jc(H) characteristics in high-Tc superconductors are dominated by the portion of strongly coupled regions at the grain boundaries. As a result of inhomogeneity of the grain boundaries, the coupling strength varies and causes a distribution of Jgo (inter-granular Jc) related to the fluctuations of the superconducting order parameter at the boundary area. We also discuss formation mechanisms of strongly coupled regions in bulk materials, and we present inductive methods that can be used to determine the coupling strength of a material.