Influence of splayed distributions of columnar defects on the pinning properties of Bi-2212 single crystals

Different configurations of tilted columnar defects have been introduced in Bi2Sr2CaCu2O8 (Bi-2212) superconducting single crystals by irradiation with 6 GeV Pb ions. All the defects are tilted of the same value theta(i) from the (c) over right arrow axis and are introduced along one (n = 1), two (n = 2) or three (n = 3) directions, symmetrically distributed around the (c) over right arrow axis (for n > 1). Several values of theta(i) have been studied: 45 degrees, 60 degrees and 75 degrees. Magnetic measurements were performed with (H) over right arrow//(c) over right arrow. A crossover is observed between a low temperature regime, where the pinning efficiency is the same whatever n, and a high temperature regime where the pinning efficiency is strongly reduced for n = 1. This transition can be related to a difference in the accommodation abilities of vortices to defects. For symmetric distributions (n > 1), accommodation persists up to high temperatures in a wide field range, contrary to n = 1. Another configuration, with two directions nonsymmetrically distributed around (c) over right arrow was also investigated in order to distinguish the influence of the symmetry from the influence of angular dispersion. (C) 1998 Elsevier Science B.V. All rights reserved.