AC SUSCEPTIBILITY STUDIES OF THE VORTEX LATTICE MELTING TRANSITION INCLEAN, DETWINNED YBA2CU3O6.92 SINGLE-CRYSTALS

We present data from a.c. susceptibility and resistivity measurements in an applied d.c. magnetic field on clean detwinned YBa2Cu3O6.92 Sing le crystals. Our investigations have revealed an extremely sharp ampli tude and frequency independent onset of both the in phase and out of p hase a.c. susceptibility components. For fields below H=6T the positi on of this onset in H-T plane agrees with the melting transition obtai ned from resistivity measurements on the same crystal. At H greater th an or equal to H this transition becomes criteria dependent. We found that H accurately correlates with the multi-critical point obtained from resistivity measurements.