First-order pre-melting transition of vortex lattices

Vortex lattices in the high-temperature superconductors undergo a first-order phase transition that has thus far been regarded as melting from a solid to a liquid. We point out an alternative possibility of a two-step process in which there is a first-order transition from an ordinary vortex lattice to a soft vortex solid followed by another first-order melting transition from the soft vortex solid to a vortex liquid. We focus on the first step. This pre-melting transition is induced by vacancy and interstitial vortex lines. We obtain good agreement with the experimental transition temperature against field, latent heat, and magnetization jumps for YBa2Cu3O7-delta and Bi2Sr2CaCu2O8.