SPLAYED CONFIGURATIONS OF COLUMNAR DEFECTS IN YBA2CU3O7, (TI,BI)SR2CACU2O7 AND BI2SR2CACU2O8 SINGLE-CRYSTALS - STUDY OF THE ENHANCED PINNING EFFICIENCIES FOR DIFFERENT CHARACTERISTIC ANGLES

Splayed configurations of columnar defects with different characterist ic angles theta(i) relative to the c-axis have been achieved in single crystals of YBCO, (Tl, Bi)-1212 and Bi-2212. Their pinning efficienci es for H parallel to c have been studied by magnetic measurements. An increased pinning efficiency consistent with the splayed model of Hwa et al. [Phys. Rev. Lett. 71 (1993) 3545] has been observed with theta( i) = 10 degrees for YBCO and (T1, Bi)-1212. On the other hand, the eff ect is detrimental for the more anisotropic compound Bi-2212. Moreover , higher angles up to theta(i) = 75 degrees have yielded further pinni ng improvements in all the compounds for high temperatures and low fie lds, but these latter results are more delicate to interpret.