Phase diagram of in the presence of columnar defects

Using a combination of high-precision torque and SQUID magnetometry we investigate the irreversibility line of single-crystalline (Formula presented) 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 (Formula presented) 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.