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

We report magnetization measurements on clean and untwinned YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_{6.95}$ 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 ${0.8k}_B$ per vortex per Cu${\mathrm{O}}_2$ double layer at 10 kOe and ${0.6k}_B$ per vortex per Cu${\mathrm{O}}_2$ double layer at 40 kOe, in reasonable agreement with theoretical calculations.