Granularity of the Critical Current Pattern and Degradation of Macroscopic Material Properties due to Mesoscopic Material Inhomogeneities

摘要

Because the defect structure of superconducting materials for cryomagnetic applications (e.g., melt textured YBCO) is characterized by length scales of several orders of magnitude, the critical current density must be considered, in general, as a space- dependent function j_c = j_c(r) on a mesoscopic length scale L. It is shown that any regular critical current pattern for j_c(r) = const is unstable against such mesoscopic fluctuations of j_c(r). In particular, there appears a granular current pattern due to the formation of local current loops on the characteristic length scale L while, at the same time, macroscopic material properties (remanent magnetic momentum, trapped induction) show a degradation. Analytical results within a consistent model description are presented.