Enhanced Pinning with Controlled Splay Configurations of Columnar Defects; Rapid Vortex Motion at Large Angles

Orders-of-magnitude enhancements of persistent currents J are reported in $\mathrm{YBa}{}_2{\mathrm{Cu}}_3\mathrm{O}{}_{7-\delta }$ with columnar defects arranged in a variety of splayed configurations. The largest J is obtained for a planar distribution $P_{\mathrm{pl}}\left(\Theta \right)$, with a splay angle ${\Theta }_{\mathrm{opt}}=\pm 5°$. A comparison of $P_{\mathrm{pl}}\left(\Theta \right)$ and a Gaussian distribution $P_{\mathrm{G}}\left(\Theta \right)$ suggests that pinning by the latter is controlled by large-angle tails of the Gaussian which appear to enhance thermal creep rate. Numerical simulations confirm the existence of the regimes where vortex motion is promoted rather than suppressed by splay.