COMPARATIVE AC SUSCEPTIBILITY AND RESISTIVITY STUDIES OF THE VORTEX LATTICE MELTING TRANSITION IN DETWINNED YBA2CU3O7-DELTA SINGLE-CRYSTALS

We have studied the transition from the vortex liquid to the vortex so lid state in clean, detwinned YBa2Cu3O7-delta single crystals in field s up to 12 T using AC susceptibility and resistivity measurements. For mu(0)H < 7 T a sharp drop in the resistivity at a temperature T-m (wh ich has been shown to be a signature of a first order phase transition [1,2]) was found to be accompanied by an AC field independent abrupt onset of both chi' and chi '' components. The AC susceptibility data s uggest that just below T-m there is a transition region where the liqu id and solid phases coexist. The width of the transition region was fo und to be sample dependent and can be related to structural inhomogene ity. Above the multicritical field mu(0)H(mc) = 7 T simultaneous broad ening of both the rho(T) drop and the chi ''(T) onset show that the fi rst order melting transition is suppressed. (C) 1998 Elsevier Science B.V.