SEPARATION OF THE IRREVERSIBILITY AND MELTING LINES IN BI2SR2CACU2O8 CRYSTALS

Bi2Sr2CaCu2O8 crystal was polished into a prism shape that eliminates the geometrical barrier and results in a fully reversible magnetizatio n at T greater than or similar to 76 K. The vortex-lattice melting was determined independently by the magnetization step at the transition. At elevated temperatures the irreversibility line (IL) lies in the vo rtex-solid region significantly below the melting line, whereas at low er temperatures the IL is high within the vortex-liquid phase and is d ue to surface barriers. The results clearly demonstrate that the IL an d melting stem from different and unrelated physical mechanisms.