As an effective method for improving the critical current density (J_c) of superconductors, heavy-ion irradiation is widely used [1]. Columnar defects (CDs) are introduced by heavy-ion irradiations, which are strong pinning centers to suppress vortex motion thus to enhance J_c. Other than J_c enhancement, an anomalous peak effect at ~1/3B_Φ (B_Φ is the dose equivalent matching field, and B_Φ = 1 T corresponds to 5×10¹⁰ cm^-2 CDs.) was observed in a twinned YBa₂Cu₃O_7-δ single crystal when the magnetic field was applied parallel to CDs (θ_H = θ_CD; θ_H and θ_CD are angles from the c-axis to the applied field and CDs, respectively.) [2]. In their experiments [2], for distinguishing the effect of CDs introduced by heavy-ion irradiation from that of twin boundaries parallel to the c-axis on J_c, they intentionally introduced tilted CDs (θ_CD = 30º). If we can find a superconductor with no correlated defects, the peak effect can be observed without tilting CDs. The conventional low-temperature superconductor NbSe₂ is such a good candidate. Contrary to our expectation, however, the peak effect was not observed when CDs were introduced parallel to the c-axis (θ_CD = 0º) and the field was applied along the c-axis (θ_H = 0º) as shown in Fig. 1(a). Although vortex structures for the two cases (θ_H = θ_CD = 30º and θ_H = θ_CD = 0º) are expected to be very similar intuitively, the results are very different. On the other hand, we have reported the presence of peak effect in NbSe₂ with splayed CDs, where the splay angle is as small as ±5º [3]. In the present study, we introduced splayed CDs in NbSe₂ with θ_CD = ±1º and observed a similar peak effect by applying magnetic field parallel to the c-axis (θ_H = 0º) as shown in Fig. 1(b). The fact that only ±1º splay of CDs induced behavior different from the case of θ_CD = 0º is unexpected. To find possible explanation for the difference between these two cases, studies on the θ_H dependence of J_c are necessary. In order to measure θ_H dependence of J_c efficiently and precisely, we modified the horizontal rotator for SQUID magnetometer (MPMS5-XL) and used small Hall probes set on the rotator to detect the local magnetic field from the sample and external field. We will present the θ_H dependence of J_c on samples with CDs parallel to the c-axis and with splayed CDs (θ_CD = ±1º), and discuss possible mechanism of the peak effect.

[1] T. Tamegai et al., Supercond. Sci. Technol. 25, 084008 (2012).
[2] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
[3] W. Li et al., J. Phys. Conf. Ser. 2323, 012017 (2022).

Fig. 1. Magnetic field dependence of J_c for 2H - NbSe₂ single crystals irradiated by 320 MeV Au (a) along the c-axis (B_Φ = 4 T, θ_CD = 0º) and (b) from two directions symmetrically (Total B_Φ = 4 T, θ_CD = ±1º ).

Keywords: NbSe2, heavy-ion irradiation, peak effect