with artificial pinning centers*

Irradiations of swift particles into superconductors modify their vortex states and lead to the enhancement of critical current density, J_c, via creation of point defects (PDs) or columnar defects (CDs). Such an enhancement of J_c was demonstrated in cuprate superconductors [1] as well as in conventional [2] and iron-based superconductors [3]. However, how the J_c is enhanced depends strongly on the target material and ion species, energy, and density of swift particles. In some cases, J_c is monotonically enhanced, while in other cases, non-monotonic magnetic field dependence of J_c, peak effect, is observed after the irradiation. In the present study, we compare the effect of PDs and CDs on the J_c - H behavior of NbSe₂. With increasing the density of PDs by proton irradiation, the field range for the peak effect close to H_c2 broadens significantly. In the case of CDs, the J_c - H behavior changes sensitively as a function of the direction of CDs and magnetic field with respect to the c-axis.
[1] L. Civale et al., Phys. Rev. Lett. 67, 648 (1991).
[2] P. Bauer et al., Europhys Lett. 23, 585 (1991).
[3] Y. Nakajima et al., Phys. Rev. B 80, 012510 (2009).