Phase Transitions and Vortex Line Entanglement in a Model High Temperature Superconductor

Monte Carlo simulations of the uniformly frustrated 3d XY model are used to model vortex line fluctuations in high temperature superconductors in an applied magnetic field. We find two distinct phase transitions. At a lower T_{c\perp}, the vortex lattice melts and coherence is lost in planes perpendicular to the magnetic field. At a higher T_{cz}, a vortex tangle percolates throughout the system, and coherence is lost parallel to the magnetic field. Cooling below T_{cz}, high energy barriers for vortex line cutting lead to an entangled glassy state. Figures available upon request to [email protected]: 20 pages, 15 figures, RevTex3.0, UR-93-ST0