Mean-Field Theory of an Axial ChiralXYModel with Anisotropy

Abstract

The finite-temperature phase diagram is studied for an axial chiral X Y model with p -fold anisotropy in which the nearest neighbor spins along a particular direction are coupled through the chiral interaction while those in a plane perpendicular to this direction are coupled ferromagnetically; the ground-state of the model is known to have a complicated phase diagram characterized by the devil's staircase. By using the mean-field theory, the cases of p = 2, 3, 4 and 6 are investigated. The global phase diagram is constructed by employing the minimization eigenvalue method to find the spin configuration that minimizes the free energy. Certain properties of the phase diagram are studied analytically. The structure of the finite-temperature phase diagram is found to change qualitatively with p , though the qualitative feature of the ground-state phase diagram is known to be independent of p ≥2.