FIRST-ORDER MELTING AND DYNAMICS OF FLUX LINES IN A MODEL FOR YBA2CU3O7-DELTA

We have studied the statics and dynamics of flux lines in a model for YBa2CU3O7-delta using both Monte Carlo simulations and Langevin dynami cs. The lines are assumed to be flexible but unbroken in both the soli d and Liquid states. For a clean system, both approaches yield the sam e melting curve, which is found to be weakly first order with a heat o f fusion of about 0.02k(B)T(m) per vortex pancake at a field of 50 kG. The time-averaged magnetic field distribution experienced by a fixed spin is found to undergo a qualitative change at freezing, in agreemen t with NMR and muon spin resonance experiments. The calculations yield , not only the field distribution in both phases, but also an estimate of the measurement time needed to distinguish these distributions: We estimate this time as greater than or equal to 0.5 mu sec. The magnet ization relaxation time in a clean sample slows dramatically as the te mperature approaches the mean-field upper critical field line H-c2(T) from below. Melting in the clean system is accompanied by a proliferat ion of free disclinations and a simultaneous disappearance of hexatic order. Just below melting, the defects show a clear magnetic-field-dep endent two- to three-dimensional crossover from long disclination line s parallel to the c axis at low fields, to two-dimensional ''pancake'' disclinations at higher fields. Strong point pins produce an energy v arying logarithmically with time. This Int dependence results from slo w annealing out of disclinations in disordered samples. Even without p ins, the model gives subdiffusive motion of individual pancakes in the dense liquid phase, with mean-square displacement proportional to t(1 /2) rather than to t as in ordinary diffusion. The calculated melting curve and many dynamical features agree well with experiment.