Vortex Dynamics and Critical Current Densities in 90 K-phase and 60 K-phase YBa2Cu3O7-δ with Splayed Columnar Defects
*Jiachen Wang1, Nozomu Ito1, Sunseng Pyon1, Teruzaku Nishizaki2, Tsuyoshi Tamegai1

Enhancement of critical current density (Jc) by the introduction of columnar defects (CDs) via heavy-ion irradiation was demonstrated in cuprate superconductors [1]. Further enhancement of Jc was also demonstrated by splaying the direction of CDs [2]. 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 [2]. However, similar drops in Jc have not been observed in iron-based superconductors with CDs [3]. 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 [2]. Also, a sharp increase of Jc at θCD = 5˚ shown in Figure. 1 (b) is reproduced like that reported in Ref. [2]. 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.

[1] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
[2] L. Krusin-Elbaum et al., Phys. Rev. Lett. 76, 2563 (1996).
[3] 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