1 5 Ju n 20 05 Transitions and crossover phenomena in fully frustrated XY systems

We study the two-dimensional fully frustrated XY (FFXY) model and two related models, a discretization of the Landau-Ginzburg-Wilson Hamiltonian for the critical modes of the FFXY model and a coupled Ising-XY model, by means of Monte Carlo simulations on square lattices L2, L . 103. We show that their phase diagram is characterized by two very close chiral and spin transitions, at Tch > Tsp respectively, of the Ising and Kosterlitz-Thouless type. At Tch the Ising regime sets in only after a preasymptotic regime, which appears universal to some extent. The approach is nonmonotonic for most observables, with a wide region controlled by an effective exponent νeff ≈ 0.8. PACS Numbers: 74.81.Fa, 64.60.-i, 64.60.Fr, 75.10.Hk Typeset using REVTEX