Improved Pinning Morphology in HTS with Order of Magnitude Increase in Jc and Pinned Field

Abstract : In 2004 most scientists working to increase Jc in HTS believed that continuous columnar pinning centers (CCPCs) were ideal. This was based upon elegant ionic radiation studies yielding increased Jc. Our contrary evidence augered for performing an experiment comparing Jc for a wide range of pinning center (PC) continuities and diameters (including CCPCs) produced by high energy ions. (Increased discontinuity correlated with lower diameter.) Multiple-in-line-damage (MILD) PCs had not been systematically studied before this. Results showed an increase in Jc by a factor of 17, for discontinuity near 67%, with PC diameter ~ 6.8 nm and fluence of 10*12/cm*2. Analysis, assuming that the Jc enhancement was caused by the over 10-fold decrease in MILD PC/ CCPC damage, matched the data well except for a Jc fishtail effect, and Jc increase vs. fluence. Analysis of the discontinuities showed they enhanced vortex wandering, and produced entanglement which increased at higher fields, resulting in a fishtail. Wandering vortices jump from gaps in ion tracks to adjacent continuous tracks, diminishing pinning losses. At higher fluence, closer tracks leave less unpinned vortex length. Thus, discontinuities encourage entanglement while restoring pinned vortex length. Both effects increase vortex binding and Jc, while CCPCs suppress entanglement.