The in- and out-of-plane resistivities of untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ show that at the melting temperature of the vortex lattice ${\mathrm{T}}_{\mathrm{m}}$(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 transition. The vortex liquid just above ${\mathrm{T}}_{\mathrm{m}}$(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.

• Received 12 February 1997

DOI:https://doi.org/10.1103/PhysRevB.55.14156