High-Tc Superconductors in Applied Magnetic Fields Parallel to the CuO Planes: First Order Transition with Slow Onset of Resistivity

The three dimensional uniformly frustrated XY model is used as a model of a high temperature superconductor in an applied magnetic field parallel to the CuO-planes. Through Monte Carlo simulations with anisotropy ${\eta }^2=10$ on large lattices we find evidence for a first order transition. Earlier simulations and theoretical treatments are discussed and the experimentally found smooth onset of resistivity is suggested to be due to a large potential barrier against vortex line motion above $T_c$ present for perfect alignment of the applied field.