Nature of the Low Field Transition in the Mixed State of High Temperature Superconductors

TL;DR

This study numerically investigates the low-field transition in high-temperature superconductor vortex lattices, revealing a weakly first-order melting transition and a sharp crossover possibly indicating a phase transition near a specific temperature.

Contribution

It introduces a combined static and dynamic model of vortex behavior, highlighting a transition or crossover distinct from flux lattice melting in high-temperature superconductors.

Findings

Vortex lattice melts into a line liquid at low fields.

Flux flow resistivity near the crossover resembles experimental data.

Sharp increase in vortex density correlates with magnetization jump.

Abstract

We have numerically studied the statics and dynamics of a model three-dimensional 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 $T_{\ell} > T_m$ which develops at the same temperature as an infinite vortex tangle. The calculated flux flow resistivity in various geometries near $T=T_{\ell}$ closely resembles experiment. The local density of field induced vortices increases sharply near $T_\ell$, corresponding to the experimentally observed magnetization jump. We discuss the nature of a possible transition or crossover at $T_\ell$(B) which is distinct from flux lattice melting.