Critical-current anisotropy due to inclined and crossed linear defects.

Abstract

Parallel linear defects which are inclined to the sample surface normal and crossed linear defects are introduced into ${\mathrm{DyBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals by high-energy heavy-ion irradiation. The dependence of flux penetration on temperature, fluence, and applied external magnetic field is investigated before and after irradiation. For crossed linear defects much higher pinning than for parallel defects with the same density is observed for both materials. This finding is attributed to the intersections of linear defects which allow the flux lines to be more effectively pinned than by parallel defects. In this case flux-line depinning occurs by kink-pair nucleation in the sample volume as in the Bose-glass model. From parallel linear defects the flux lines may be more easily depinned by nucleation of surface kinks. The observed low anisotropy ${\mathit{j}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathit{j}}_{\mathrm{\ensuremath{\perp}}}$\ensuremath{\approxeq}2 of the critical current flowing parallel or perpendicular, respectively, to the inclination plane of the defects in parallel-irradiated ${\mathrm{DyBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ is explained by different geometrical arrangment of defects and flux lines in the directions of flux-line motion. During field decrease the anisotropy is found to disappear in samples with parallel linear defects. For crossed defects the anisotropy ${\mathit{j}}_{\mathrm{\ensuremath{\parallel}}}$/${\mathit{j}}_{\mathrm{\ensuremath{\perp}}}$\ensuremath{\approxeq}2 reverses in decreasing field since the flux lines are effectively pinned only in the inclination plane of the linear defects. In ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals the flux penetration is isotropic; this is understood from the pancake-vortex model. The different observations in ${\mathrm{DyBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ single crystals are in good agreement with a recently given scaling approach.