Summation of Flux Pinning by Columnar Defects Tilted at Different Angles in YBCO Thin Films

Angular behaviors of critical current density J _c and n-values at 77 K, 1 T were investigated in YBCO films with columnar defects (CDs) tilted at small angles to the ab-plane before and after the additional introduction of CDs at different angles, where CDs were installed by heavy-ion irradiations. The enhanced J _c at B || ab by the initial irradiation at θ _i = ±85° relative to the c-axis was reduced by the additional irradiation at θ _i = ±75°, whereas the J _c was increased at the intermediate angle between the two irradiation angles. The addition of CDs at θ _i = 0° also lowered the J _c around B || ab, which were enhanced by the initial irradiation angles of θ _i = ±80°. The n-values, by contrast, were hardly lowered in any direction of magnetic field by the second irradiation: the n-value seems to be determined by a simple sum of the pinning interactions of CDs tilted at different angles. These results suggest that the flux pinning is improved in each direction of the tilted CDs, without impeding the flux pinning in the other CD direction each other. The additional introduction of CDs, however, induces both the reduction of the superconducting percolation path and the irradiation damage to the ab-correlated PCs, resulting in the reduction of J _c .