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

Abstract

Orders-of-magnitude enhancements of persistent currents J are reported in $\mathrm{YBa}{}_{2}{\mathrm{Cu}}_{3}\mathrm{O}{}_{7\ensuremath{-}\ensuremath{\delta}}$ with columnar defects arranged in a variety of splayed configurations. The largest J is obtained for a planar distribution ${P}_{\mathrm{pl}}(\ensuremath{\Theta})$, with a splay angle ${\ensuremath{\Theta}}_{\mathrm{opt}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$. A comparison of ${P}_{\mathrm{pl}}(\ensuremath{\Theta})$ and a Gaussian distribution ${P}_{\mathrm{G}}(\ensuremath{\Theta})$ 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.