First-order vortex-lattice melting transition in YBa₂Cu₃O_7-δ near the critical temperature detected by magnetic torque

High-resolution magnetic-torque studies on an untwinned YBa2Cu3O7-δ single crystal near its critical temperature Tc reveal that the first-order vortex-lattice melting transition (VLMT) persists at least up to 0.5 K below Tc.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 Tc.© 1998 The American Physical Society.