First-Order Vortex-Lattice Melting Transition inYBa2Cu3O7−δnear the Critical Temperature Detected by Magnetic Torque

High-resolution magnetic-torque studies on an untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 $T_c$.