PHASE-DIAGRAM OF BI2.15SR1.85CACU2O8+DELTA IN THE PRESENCE OF COLUMNAR DEFECTS

Using a combination of high-precision torque and SQUID magnetometry we investigate the irreversibility line of single-crystalline Bi2.15Sr1. 85CaCu2O8+delta defected with irradiation-induced columnar pins. We fi nd that the irreversibility line cannot be identified with a shifted m elting Line but rather follows the crossover line B-rb(T) separating t he single-vortex strong pinning regime from the collective weak pinnin g regime. Good agreement is found with a theoretical analysis accounti ng for the strong layering of the material.