Phase-ordering kinetics with external fields and biased initial conditions

The late-time phase-ordering kinetics of the O(n) model for a nonconserved order parameter are considered for the case where the O(n) symmetry is broken by the initial conditions or by an external field. An approximate theoretical approach, based on a ``Gaussian closure'' scheme, is developed, and results are obtained for the time dependence of the mean order parameter, the pair correlation function, the autocorrelation function, and the density of topological defects [e.g., domain walls (n=1), or vortices (n=2)]. The results are in qualitative agreement with experiments on nematic films and related numerical simulations on the two-dimensional XY model with biased initial conditions.