FINITE VORTEX CORRELATION IN THE C DIRECTION IN YBA2CU3O7-DELTA ABOVETHE FIRST-ORDER MELTING TRANSITION

The in- and out-of-plane resistivities of untwinned YBa2Cu3O7-delta sh ow that at the melting temperature of the vortex lattice T-m(H), the c -axis vortex correlation length has a finite value, demonstrating that the first-order transition is a true melting and not a decoupling tra nsition. The vortex liquid just above T-m(H) has a correlation length in the field direction on the order of a micron. The in-plane I-V char acteristics in the liquid state show non-Ohmic behavior, providing evi dence that thermal fluctuations dominate the response even at the firs t-order transition.