One of the famous pinning phenomena in type-II superconductors is the synchronization peak effect which occurs near H_c2. This phenomenon has a renewed interpretation based on the order-disorder transition of vortices [1]. By introducing artificial defects through chemical doping, the peak effect is found to be more pronounced. Using the 3 MeV proton irradiation which is known as one of effective physical ways to introduce point defects in superconductors, the enhancement of the peak effect is also observed. Other than the peak effect related to order-disorder transition, another peak effect which is induced by columnar defects (CDs) is often reported. For example, in YBa₂Cu₃O₇ introduced with tilted CDs, an anomalous peak at ~1/3B_Φ (B_Φ: dose equivalent matching field) was observed [2]. A similar peak effect has also been observed in iron-based superconductors with which CDs are introduced from two symmetric directions with respect to the c-axis at angles of ~ ±15^º [3]. Recently, a similar peak effect at ~1/5B_Φ has also been reported in NbSe₂, which is known as one of the representative conventional layered superconductors, with tilted CDs [4]. To understand how the configuration of CDs affects thepeak effect in NbSe₂, three kinds of CDs are introduced in NbSe₂ single crystals by particle irradiations. Prominent peak effects in magnetization are observed in samples with tilted CDs and splayed CDs as shown in Fig. 1(a). However, in the simplest case of CDs parallel to the c-axis, there is no peak effect as shown in Fig. 1(a). How a small tilt angle of CDs affects the appearance of the peak effect is a serious challenge. Similar measurements for samples with different θ_CD (angle between c-axis and CDs) demonstrate that J_c and the peak field show systematic but nontrivial θ_CD dependence as shown in Fig. 1(b). We try to answer these behaviors of the peak effect through systematic measurements on samples with different θ_CD, θ_H (angle between external field and c-axis), and B_Φ. Possible origins for these peak effects are discussed.

[1] Y. Paltiel et al., Phys. Rev. Lett. 85, 3712 (2000).
[2] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
[3] A. Park et al., Phys. Rev. B 97, 064516 (2018).
[4] S. Eley et al., Sci. Rep. 8, 13162 (2018).

Fig. 1. (a) Magnetic hysteresis loops for 2H-NbSe₂ single crystals irradiated by 320 MeV Au with parallel, tilted, and splayed columnar defects (total B_Φ = 2 T). (b) Critical current density for 2H-NbSe₂ single crystals introduced with splayed columnar defects with various θ_CD (total B_Φ = 4 T).

Keywords: NbSe2, artificial defects, peak effect