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FirstOrder Phase Transition with Breaking of Lattice Rotation Symmetry in ContinuousSpin Model on Triangular Lattice
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Abstract
Using a Monte Carlo method, we study the finitetemperature phase transition in the twodimensional classical Heisenberg model on a triangular lattice with or without easyplane anisotropy. The model takes account of competing interactions: a ferromagnetic nearestneighbor interaction J_{1} and an antiferromagnetic third nearestneighbor interaction J_{3}. As a result, the ground state is a spiral spin configuration for 4 < J_{1}/J_{3} < 0. In this structure, global spin rotation cannot compensate for the effect of 120° lattice rotation, in contrast to the conventional 120° structure of the nearestneighbor interaction model. We find that this model exhibits a firstorder phase transition with breaking of the lattice rotation symmetry at a finite temperature. The transition is characterized as a Z_{2} vortex dissociation in the isotropic case, whereas it can be viewed as a Z vortex dissociation in the anisotropic case. Remarkably, the latter is continuously connected to the former as the magnitude of anisotropy decreases, in contrast to the recent work by Misawa and Motome [J. Phys. Soc. Jpn. 79 (2010) 073001] in which both the transitions were found to be continuous.
Journal
 J Phys Soc Jpn

J Phys Soc Jpn 80(7), 07400807400810, 20110715
The Physical Society of Japan