1ST-ORDER PHASE-TRANSITION AT THE IRREVERSIBILITY LINE OF BI2SR2CACU2O8+DELTA

Magnetization and susceptibility measurements show a first order phase transition in the magnetic flux structure at the irreversibility line of Bi2Sr2CaCu2O8+delta single crystals, in the field range 0 < H < 360 Oe. The resistivity in the c direction drops 5 orders of magnitude in less that 0.1 K, and there is a discontinuous change in the magnetization at the same temperature. The change in magnetization together with the Clausius-Clapeyron equation gives a variation of entropy of 0.06k(B) per vortex layer, at the transition. Above 360 Oe the irreversibility line has the known frequency dependence.