Effects of Asymmetric Splayed Columnar Defects on the Anomalous Peak Effect in Ba0.6K0.4Fe2As2

In order to enhance the critical current density (J(c)) of superconductors, introduction of columnar defects (CDs) through swift-particle irradiation is effective. By dispersing the direction of CDs (splayed CDs), not only further enhancement of Jc has been confirmed but also an anomalous peak effect (APE) in Jc at a certain magnetic field determined by the irradiation dose was observed. It has been proposed that the APE arises from the suppression of kink motion and/or the effects of vortex entanglement in systems with splayed CDs. In this study, we measure Jc properties of optimally K-doped Ba-122-type iron-based superconductor Ba0.6K0.4Fe2As2 single crystals with splayed CDs that are introduced asymmetrically with respect to the c-axis by irradiating 2.6 GeV U ions. We discuss the significance of the average direction of splayed CDs to the APE. We also discuss the relationship between the APE and the entanglement of vortices.