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

Abstract

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.