Effects of particle irradiations on vortex states in iron-based superconductors

Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (J(c)) and vortex dynamics have been systematically studied. It is found that J(c) is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of J(c), in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.