Influence of Defect Morphology on Absolute J_c Value in Heavy-Ion Irradiated REBCO Films

A variety of morphologies of irradiation defects ranging from columnar defects (CDs) to spherical ones were introduced into NdBa2Cu3Oy thin films by using three kinds of heavy-ion beams. The highest critical current density J(c) was obtained for the 50-MeV Kr irradiation at the matching field of B-phi = 10 T, where spherical defects were installed. In contrast, J(c) of the 200 MeV Xe irradiation which produces elongated CDs continues to increase only up to B-phi = 5 T, resulting in lower value. Interestingly, the volume fraction of the spherical defects at B-phi = 10 T was about 10 vol.%, which is almost the same as that of elongated CDs at B-phi = 5 T. These results suggest that spherical defects provide weak elementary pinning force, but can realize the introduction of much higher density without suppressing the superconducting percolation path at the same volume fraction condition. The systematic segmentation of CDs also enabled us to reveal the influence of the morphology of pinning centers on thermal relaxation rate of J(c).