First-order vortex-lattice melting transition in YBa2Cu3O7-δ near the critical temperature detected by magnetic torque

High-resolution magnetic-torque studies on an untwinned YBa2Cu3O7-delta Single crystal near its critical temperature T-c reveal that the first-order vortex-lattice melting transition (VLMT) persists at least up to 0.5 K below T-c. The associated sharp discontinuity in magnetization is detectable even at temperatures where the torque signal deviates from mean-field behavior due to fluctuations. The magnetic irreversibility at the VLMT can be suppressed by applying a weak transverse ac magnetic field. This offers the possibility of separating the irreversibility line from the melting line near T-c.