Enhancement of critical current density (Jc) by the introduction of columnar defects (CDs) via heavy-ion irradiation was demonstrated in cuprate superconductors . Further enhancement of Jc was also demonstrated by splaying the direction of CDs . In the case of 90 K-phase YBa2Cu3O7-δ with splayed CDs, the enhanced Jc drops suddenly at around 40 K, which is interpreted to be caused by the excitation of double kinks traversing neighboring CDs . However, similar drops in Jc have not been observed in iron-based superconductors with CDs . In YBa2Cu3O7-δ, in addition to the optimal 90 K-phase, where the superconductivity dominates, there is another stable 60 K-phase, where charge-density-wave competes with the superconductivity. In this study, we irradiated the 90 K-phase YBa2Cu3O7-δ by both 800 MeV Xe and 2.6 GeV U ions, while the 60 K-phase YBa2Cu3O7-δ were irradiated by 800 MeV Xe ions. Splayed CDs were introduced into those YBa2Cu3O7-δ crystals with different splay angles. After the irradiation, magnetic properties were measured by a SQUID magnetometer, and the angle and temperature dependences of Jc were evaluated by using the Bean model.
Figure. 1 (a) shows the temperature dependences of Jc at H = 10 kOe in 2.6 GeV U irradiated (BΦ = 2 T + 2 T) 90 K-phase YBa2Cu3O7-δ with different splay angles (θCD). In each condition, Jc shows a step-like decrease at around 35 K, which is consistent with the previous study . Also, a sharp increase of Jc at θCD = 5˚ shown in Figure. 1 (b) is reproduced like that reported in Ref. . We will also characterize the Jc behavior of 60 K-phase YBa2Cu3O7-δ with splayed CDs, and compare the results with those in 90 K-phase YBa2Cu3O7-δ to reveal properties of kink motions and vortex dynamics in superconductors with splayed CDs.
 L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
 L. Krusin-Elbaum et al., Phys. Rev. Lett. 76, 2563 (1996).
 Y. Nakajima et al., Phys. Rev. B 80, 012510 (2009).
Figure 1. (a) Temperature dependence of Jc at 10 kOe in 2.6 GeV U irradiated (BΦ = 2 T + 2 T) 90 K-phase YBa2Cu3O7-δ with various splay angles. (b) Splay angle dependence of Jc at 5 K and 10 kOe picked up from the data in (a).
Keywords: Cuprate superconductors, Critical current density, Pinning properties, Columnar defects