Columnar defects (CDs) were proved to enhance the critical current density (J_c) in cuprate superconductors [1], and further enhancement is reported by splaying the direction of CDs [2]. In the case of Ba_1-xK_xFe₂As₂, the J_c enhancement also occurs with a non-monotonic field dependence, namely the anomalous peak effect [3,4].

In the present study, Ba_0.6K_0.4Fe₂As₂ crystals were irradiated by 2.6 GeV U ions from two directions, each with a dose-equivalent magnetic field B_Φ = 4 T. We controlled splay angles (θ_CD), which are angles between incident directions and the c-axis, by tilting the ab-plane of samples during the irradiation. By introducing splayed CDs, which break in-plane symmetry, J_c is expected to have an in-plane anisotropy [5]. So, the two components J_para and J_perp that are parallel and perpendicular to the splay plane, respectively, need to be figured out to determine which component contributes more to the anomalous peak effect. The magneto-optical (MO) imaging can be implemented to determine them but requires observation under high fields in this case, which is difficult using a garnet film with a low saturation field. Thus, the MO imaging was only used for the detection of macroscopic defects. The in-plane anisotropy of J_c is evaluated based on the magnetization data obtained for two separate samples with different in-plane aspect ratio with respect to the splay plane. Generally, the Bean model is used to calculate the average J_c of rectangular superconductors under an assumption of constant in-plane J_c. After picking up two rectangular pieces with different aspect ratios from the original sample (θ_CD = ± 15°), J_c in the two directions were evaluated to reveal the splayed CDs-induced anisotropy in in-plane J_c. Calculated magnetic field dependence of in-plane anisotropy of J_c, J_para / J_perp are shown in Fig. 1. It clearly shows that J_para contributes more to the anomalous peak effect at around 25 kOe.

[1] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
[2] L. Krusin-Elbaum et al., Phys. Rev. Lett. 76, 2563 (1996).
[3] A. Takahashi et al., J. Phys. Soc. Jpn. 89, 094705 (2020).
[4] A. Park et al., Phys. Rev. B 97, 064516 (2018).
[5] A. Park et al., Physica C 530, 58 (2016).
Fig. 1. Magnetic field dependence of in-plane anisotropy of Jc, Jpara / Jperp at various temperatures for Ba0.6K0.4Fe2As2 (θCD = ± 15°, BΦ = 4 T + 4 T).

Keywords: Iron-based superconductor, Critical current density, Anomalous peak effect, Columnar defects