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

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.