The Magnetic-Field Dependence of Critical Current: What We Really Need to Know

In high-temperature superconducting coated conductors, it is well known that engineered flux pinning defects and the deposition techniques themselves often lead to strong anisotropic variations in the critical current that are not necessarily aligned with the coordinate system of the wire. Nevertheless, it is common in the literature and in practice to characterize superconducting wires in terms of the field dependences of their critical current for magnetic fields applied perpendicular to the plane of the wire, with the implication that the critical current has a minimum value for this configuration. Here, we demonstrate the extent to which oversimplified interpretations of field dependencies can provide a misleading view of the actual wire performance. Samples that appear to be isotropic are shown in fact to be highly anisotropic, while minimum critical current values are seen to differ from those values obtained in the cardinal field directions. In lieu of a full angular characterization, we propose an alternative magnetic-field dependence: that of the angular minimum of the critical current. This metric can be a useful guide to efficient device design, and is also an informative way to evaluate critical current enhancements effected by compositional or processing variations.