High flux pinning efficiency by columnar defects dispersed in three directions in YBCO thin films

A systematic investigation of flux pinning by widely direction-dispersed columnar defects (CDs) in YBa₂Cu₃O_y thin films was carried out by using heavy-ion irradiation: a parallel configuration of CDs aligned along the c -axis, and two trimodal splay configurations composed of CDs crossing at 0° and ± 45° relative to the c -axis, where the splay plane defined by the three irradiation angles is perpendicular (trimodal-A) or parallel (trimodal-B) to the transport current direction. The trimodal configurations show high pinning efficiency over a wide range of magnetic field orientations compared to the parallel one at low magnetic field. In particular, trimodal-B shows the higher critical current density of the two trimodal configurations. The crossed CDs at ± 45° in the trimodal configurations provide uncorrelated flux pinning at B || c due to the large tilting angle off the c -axis, which effectively reinforce the flux pinning of CDs parallel to the c -axis. This assist effect is more remarkable for trimodal-B: a kink sliding motion of flux lines along the CDs is more effectively reduced by the splay plane, not only at B || c but also at inclined magnetic fields off the c -axis. (paper)