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 find that the irreversibility line cannot be identified with a shifted melting Line but rather follows the crossover line B-rb(T) separating the single-vortex strong pinning regime from the collective weak pinning regime. Good agreement is found with a theoretical analysis accounting for the strong layering of the material.