Nature of the low-field transition in the mixed state of high-temperature superconductors

We have numerically studied the statics and dynamics of a model three-dimensional (3D) vortex lattice at low magnetic fields. For the statics we use a frustrated 3D XY model on a stacked triangular lattice. We model the dynamics as a coupled network of overdamped resistively shunted Josephson junctions with Langevin noise. At low fields, there is a weakly first-order phase transition, at which the vortex lattice melts into a line liquid. Phase coherence parallel to the field persists until a sharp crossover, conceivably a phase transition, near T-l > T-m which develops at the same temperature as an infinite vortex tangle. The calculated flux flow resistivity in various geometries near T = T-l closely resembles experiment. The local density of field induced vortices increases sharply near T-l, corresponding to the experimentally observed magnetization jump. We discuss the nature of a possible transition or crossover at T-l(B) which is distinct from flux lattice melting.