REPLICA SYMMETRY-BREAKING INSTABILITIES IN THE 2D XY MODEL IN A RANDOM-FIELD

We study the 2D vortex-free XY model in a random field, a model for randomly pinned flux lines in a plane. We construct controlled renormalization group recursion relations which allow for replica symmetry breaking (RSB). The fixed point previously found by Cardy and Ostlund in the glass phase T<Tc is unstable to RSB. The susceptibility associated to infinitesimal RSB perturbation in the high-temperature phase is found to diverge as (T-Tc)-, when TTc+. This provides analytical evidence that RSB occurs in finite dimensional models. The physical consequences for the glass phase are discussed. © 1995 The American Physical Society.