Dimensional crossover in the quasi-two-dimensional Ising-O(3) model

摘要

We present the results of our Monte Carlo simulation of the Ising-O(3) model on two-dimensional (2D) and quasi-2D lattices. This model is an effective classical model for the stacked square-lattice J_1-J_2 Heisenberg model, where the nearest-neighbor (J_1) and next-nearest-neighbor (J_2) couplings are frustrated and we assume that J_2 is dominant. We find an Ising ordered phase in which the O(3) spins remain disordered in a moderate quasi-2D region. There is a single first-order transition for a sufficiently large 3D coupling, in agreement with a renormalization group treatment. The subtle region in which the single transition splits into two transitions is also discussed and compared against recent measurements of two very close transitions in BaFe_2As_2. Our results can provide a qualitative explanation of the experiments on ferropnictides, namely the observed sequence and orders of the structural and magnetic transitions, in terms of the ratio between the interlayer and intralayer coupling.