Discontinuity of Reversible Magnetization in Untwinned YBCO Single Crystals at the First Order Vortex Melting Transition

We report magnetization measurements on clean and untwinned YBa₂Cu₃{O}6.95$ single crystals with extremely low pinning. For the first time, an irreversibility line well below the vortex melting line has been achieved. A discontinuity in the magnetization at the vortex melting transition is observed, indicating a first order transition. The entropy change at the phase transition is 0.8kB per vortex per CuO₂ double layer at 10 kOe and 0.6kB per vortex per CuO₂ double layer at 40 kOe, in reasonable agreement with theoretical calculations.