抄録
Columnar defects (CDs) produced by heavy-ion irradiations in high-Tc superconductors such as YBa2Cu3Oy (YBCO) are the most effective pinning centers to immobilize flux lines, leading to the enhancement of the critical current density Jc. In addition, slight dispersion (splay) in the directions of CDs can further improve the Jc. In this work, YBCO thin films were irradiated using 200 MeV Xe ions at two angles ±θi relative to the c-axis in two geometries where the direction of current is perpendicular (mode A) or parallel to the splay plane (mode B), to investigate the influence of the direction of splay plane on the flux pinning properties. In a low magnetic field parallel to the c-axis, a weaker magnetic field dependence of the critical current density Jc for mode B than that for mode A was observed and the Jc for mode B with θi=±45º was larger than any other sample in this work, regardless of larger crossing angles relative to the c-axis. This is attributed to the quasi-2D flux pinning by the splay plane perpendicular to the flux line motion for mode B. The behaviour of the angular dependence of Jc significantly depends on the direction of the splay plane: a crossover from a rectangle-shaped Jc peak to double peaks occurs in the angular dependence of Jc for mode A with θi =±45º in increasing magnetic field, whereas a remarkably large and broad Jc peak appears for mode B with θi =±45º.
