Transitions and crossover phenomena in fully frustrated XY systems

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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 L x L, L=O(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 nuₑff=0.8.