Vortex pinning by different angular distributions of columnar defects in Bi2Sr2CaCu2O8

Different configurations of columnar defects tilted with respect to c have been tailored in Bi2Sr2CaCu2O8 single crystals by controlling the orientation of a heavy-ion beam: One single direction of tracks yielding parallel defects (n=1), two coplanar directions of tracks (n=2), and three directions of tracks symmetrically distributed around the c axis (n=3). The pinning efficiency has been systematically investigated by magnetic measurements with the field applied along c. We found evidence for a low-T regime where the pinning efficiency is nearly independent of it, and a high-T regime where it essentially increases with n. These features are discussed in the frame of two different but not incompatible approaches: (i) a static approach based on accommodation effects, and (ii) a dynamic approach based on elementary flux-creep processes. Finally, the further comparison of these results with angular measurements performed for n=1 strongly suggests that the symmetry of the track distribution relative to the orientation of the applied field might play a major role in the pinning efficiency of splayed configurations.