Enhancement of J_c at around 1/3B_Φ in Iron-based Superconductors with Splayed Columnar Defects

Columnar defects (CDs) are known to trap vortices very effectively, leading to a significant increase in the critical current density (J_c). In contrast to the case of parallel CDs along the c-axis, splaying the direction of columnar defects enhances J_c even further due to the suppression of kink motions. In (Ba,K)Fe₂As₂ with splayed CDs introduced by irradiating 2.6 GeV U ions, we found a novel enhancement of J_c at around 1/3B_Φ, (B_Φ: matching field) for a range of splay angle of CDs. In the present study, we compare the enhancement of J_c in (Ba,K)Fe₂As₂ with splayed CDs created by 2.6 GeV U ions with those created by 320 MeV Au ions. We find out that the enhancement of J_c always occurs at ~1/3B_Φ, while the degree of enhancement and optimum splay angle depends on the morphology of splayed CDs. We also investigate the effect of the symmetry breaking of the two sets of splayed CDs, namely for the case where the average direction of two sets of CDs is different from the c-axis and another case where doses of two sets of splayed CDs are different.