Phase diagram for a generalized fully frustrated triangular XY model.
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To better understand the phase diagram for XY spins on the fully frustrated triangular lattice, we generalize that model, giving one class of bonds a tunable strength -J, where =1 gives the conventional model. As a consequence, the sites are no longer all equivalent. Using mean-field theory, the phase diagram has been determined as a function of, the applied field H, and the temperature T. For H=0, the phase diagram is very similar to that for the generalization of Berge et al. of the fully frustrated XY square lattice model. For H 0 and -0.5<<1., a nonuniform collinear phase intervenes between two noncollinear phases (one chiral and the other nonchiral). This nonuniform collinear phase is an extension of the paramagnetic phase (which, in a field, also is nonuniform and collinear), rather than being the (spontaneous symmetry-breaking) nonuniform collinear Potts phase of the =1 Monte Carlo phase diagram. For <-0.5 this extension of the paramagnetic phase completely displaces the (low-field) chiral phase. For >1 the extension of the paramagnetic phase is absent. For =1 we conclude that the simplest explanation for the Potts phase can be obtained by comparison to the Monte Carlo phase diagram for Ising spins on this lattice. 1991 The American Physical Society.
