ASYMMETRIC PHASES IN THE MEAN-FIELD THEORY OF THE AXIAL NEXT-NEAREST-NEIGHBOR ISING-MODEL

We study the effect of the weakening of the intraplane interactions on the mean-field theory of the axial next-nearest-neighbor Ising model and show that commensurate phases, which lack the usual reflection or inversion symmetries, may be stabilized. These asymmetric phases separate commensurate phases with the same period but different symmetries, and the phase-locking angle changes continuously as a function of the temperature. We also compute the phase boundaries numerically and by a continuum approximation, and show that they have a characteristic bottleneck shape.