Vortex-line fluctuations in model high-temperature superconductors.

We carry out Monte Carlo simulations of the uniformly frustrated three dimensional XY model, as a model for vortex line fluctuations in a high Tc superconductor in an external magnetic field. A density of vortex lines of f = 1/25 is considered. We find two sharp phase transitions. The low T superconducting phase is an ordered vortex line lattice. The high T normal phase is a vortex line liquid, with much entangling, cutting, and loop excitations. An intermediate phase is found, which is characterized as a vortex line liquid of disentangled, approximately straight, lines. In this phase, the system displays superconducting properties is the direction parallel to the magnetic field, but normal behavior in planes perpendicular to the field. A detailed analysis of the vortex structure function is carried out. PACS: 74.60.Ge, 64.60-i, 74.40.+k