PHASE-DIAGRAM OF THE MIXED-STATE OF BI2SR2CACU2O8, SINGLE-CRYSTALS - FIRST-ORDER PHASE-TRANSITION AT THE IRREVERSIBILITY LINE

Magnetization and susceptibility measurements show a first order phase transition in the magnetic flux structure at the irreversibility line of Bi2Sr2CaCu2O8 single crystals, in the field range 0 < H < 360 Oe. The resistivity in the c direction drops six orders of magnitude in le ss that 0.1 K, and there is a discontinuous change in the magnetizatio n at the same temperature. The change in magnetization together with t he Clausius-Clapeyron equation gives a variation of entropy of 0.06k(B ) per vortex layer, at the transition. Above 360 Oe the H-T phase diag ram is determined by the laminar nature of superconductivity and the i rreversibility line is frequency dependent.