Finite vortex correlation in the c direction above the first-order melting transition

The in- and out-of-plane resistivities of untwinned (Formula presented)(Formula presented)(Formula presented) show that at the melting temperature of the vortex lattice (Formula presented)(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 transition. The vortex liquid just above (Formula presented)(H) has a correlation length in the field direction on the order of a micron. The in-plane I-V characteristics in the liquid state show non-Ohmic behavior, providing evidence that thermal fluctuations dominate the response even at the first-order transition. © 1997 The American Physical Society.