ANGULAR VARIATION OF PINNING NEAR THE IRREVERSIBILITY TEMPERATURE IN SINGLE CRYSTAL YBA2CU3O7-DELTA WITH SPLAYED COLUMNAR DEFECTS

We have studied vortex pinning through ac susceptibility measurements in single crystals of YBCO with columnar defects (CDs). The CDs have 0 degrees, 10 degrees and 20 degrees splay angle and average direction 10 degrees off the c-axis. By studying the angular variation we can co mpensate for the anisotropy and effects of twins, etc. Using a simple expression we can obtain the angular spread when the field direction i s outside the splay angle. An increase of pinning when the field direc tion is inside the angle defined by the CDs can be attributed to vorte x entanglement due to the splay and the suppression of the sliding of double kinks. (C) 1998 Published by Elsevier Science B.V. All rights r eserved.