Introduction of artificial defects in clean single crystals of superconductor by particle irradiations strongly suppresses the vortex motion and thus enhances the critical current density (J_c). This has been well studied through many experiments [1,2] for high-temperature unconventional superconductors. An anomalous peak effect has been observed in YBa₂Cu₃O₇ at ~1/3 of the dose equivalent matching field, B_Φ, when tilted columnar defects (CDs) were introduced [1]. Despite these extensive researches on high-temperature unconventional superconductors, there have been few studies on the effect of artificial defects in conventional superconductors. Recently, a peak effect at ~1/5 of B_Φ, has been observed in NbSe₂, which is a conventional layered superconductor, with tilted CDs [3]. To understand how the configuration of artificial defects affect thepinning mechanism in NbSe₂, effects of three particle irradiations (320 MeV Au, 800 MeV Xe, and 3 MeV proton) on NbSe₂ single crystals have been systematically studied. Pronounced peak effects in magnetization loops have been observed in some specific conditions for all the three kinds of irradiations such as irradiated by 800 MeV Xe with tilted CDs when the field is applied parallel to the CDs (Fig. 1a). However, in the simple case of CDs parallel to the c-axis with H // CDs, this peak effect is missing. How a small tilt angle of CDs affect the appearance of the peak effect is a serious challenge. We try to answer this question through systematic measurements on θ_CD dependence of the peak effect. Meanwhile, another peak effect appears for sample irradiated by 3 MeV protons (Fig. 1b). This peak effect has a different mechanism compared with the sample introduced with CDs. As the irradiation dose of protons is decreased, the peak field continuously increases towards H_c2, and smoothly connects to the order-disorder transition observed in pristine crystals.

[1] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
[2] T. Tamegai et al., Supercond. Sci. Technol. 25, 084008 (2012).
[3] S. Eley et al., Sci. Rep. 8, 13162 (2018).

Fig. 1. (a) Magnetic hysteresis loops for 2H-NbSe₂ irradiated by 800 MeV Xe with parallel and tilted columnar defects for H // CDs. (b) Critical current density as a function of magnetic field for 2H-NbSe₂ at various dose irradiated by 3 MeV protons.

Keywords: NbSe2, artificial defects, peak effect