# Field-driven transition in the Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductor with splayed columnar defects

## TL;DR

This study investigates how splayed columnar defects influence vortex pinning and critical current densities in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ superconductors, revealing a field-driven transition that enhances pinning at certain defect angles.

## Contribution

It demonstrates that splayed columnar defects induce a field-driven vortex pinning transition, improving critical current densities in iron-based superconductors.

## Key findings

- Optimal splay angle of ±5° yields highest J_c.
- Anisotropic discontinuity lines increase with splay angle.
- Non-monotonic field dependence of J_c observed with extrema at certain fields.

## Abstract

Through 2.6 GeV U irradiations, we have induced bimodal splayed columnar defects in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with splay angles, $\pm 5 ^\circ$, $\pm 10 ^\circ$, $\pm 15 ^\circ$, and $\pm 20 ^\circ$. Critical current densities through magnetization measurements were carefully evaluated, where a splay angle of $\pm 5 ^\circ$ brought about the highest $J_\mathrm{c}$. Mageto-optical images close to $T_\mathrm{c}$ indicates highly anisotropic discontinuity lines in the remnant state, and with anisotropy increasing with greater splay angles. Moreover, amongst those with splayed columnar defects, anomalous non-monotonic field dependences of $J_\mathrm{c}$ and $S$ with an extrema at some fraction of the matching field are observed. We discuss that such $J_\mathrm{c}$ enhancement arises from a field-driven coupling transition in which intervortex interactions reorganize the vortex structure to be accommodated into columnar defects, thereby increasing pinning at higher fields.