a.c. susceptibility studies of the vortex lattice melting transition in clean, detwinned YBa2Cu3O6.92single crystals

We present data from a.c. susceptibility and resistivity measurements in an applied d.c. magnetic field on clean detwinned YBa2Cu3O6.92single crystals. Our investigations have revealed an extremely sharp amplitude and frequency independent onset of both the in phase and out of phase a.c. susceptibility components. For fields below H*=6T the position of this onset in H-T plane agrees with the melting transition obtained from resistivity measurements on the same crystal. At H≥H*this transition becomes criteria dependent. We found that H*accurately correlates with the multi-critical point obtained from resistivity measurements.