Pinning by columnar defects in the anisotropic high-Tc superconductor Bi-2212

Besides the standard configuration with all the columnar defects parallel to the (c) over right arrow -axis, different splayed configurations with all the defects inclined at the same angle theta (i) from the (c) over right arrow -axis were obtained by irradiation of single crystals of the most anisotropic high-T-c superconductor Bi2Sr2CaCu2O8 with 6 GeV Pb ions. The pinning abilities of these configurations were studied by magnetic measurements performed with (H) over right arrow parallel to (c) over right arrow. We found that, in the case of this very anisotropic compound, it is not necessary to invoke a splay effect to account for the pinning properties of the splayed configurations of defects. The major role of two parameters has been seen. First, the role of the effective size of the projection of the defects in the ((a) over right arrow,(b) over right arrow) planes has been studied by introducing columnar defects of different diameters or by tilting the ion beam direction from the (c) over right arrow -axis. Second, the symmetry of the defect distribution with respect to the applied magnetic field has been shown to allow a better accommodation of the vortices to columnar defects. These two parameters can account for the observed crossover between a low-T regime and a high-T regime. This crossover is not predicted as a splay effect in the theoretical models. (C) 2000 Elsevier Science Ltd. All rights reserved.