Current-limiting mechanisms in individual filaments extracted from superconducting tapes

Many large-scale applications of high-temperature superconductors depend crucially on the ability to achieve high critical-current densities Jc (of the order of 105 A cm−2). Existing silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox (BSCCO) tapes have Jc values that come within about 25% of this target1,2,3,4, these values being limited by the fact that thesupercurrent flows percolatively around barriers that occur over many length scales5. To elucidate the nature of these barriers, we have measured the transport properties of individual filaments extracted from very-high-Jc multifilament tapes6. We find that Jc for individual filaments reaches at least 8× 104 A cm−2—about 50% higher than the average value over the whole cross-section of the wire. Although we injected the current to flow along the crystallographic a–b planes of the material, we found that all filaments possessed local characteristics of c-axis transport, indicating the presence of occasional nanometre- to micrometre-scale barriers at basal-plane-faced grain boundaries. An independent and much larger limiting influence on the critical current comes from unhealed cracks produced by deformation during the processing of the wires. These results provide direct evidence that better processing methods aimed at improving the c-axis alignment and at inhibiting residual cracks should raise the accessible Jc values towards those needed for applications.