On the low-temperature phase of the three-state antiferromagnetic Potts model on the simple cubic lattice

The three-state antiferromagnetic Potts model on the simple cubic lattice is investigated using the cluster variation method in the cube and the star-cube approximations. The broken-sublattice-symmetry phase is found to be stable in the whole low-temperature region, contrary to previous results obtained using a modified cluster variation method. The tiny free energy difference between the broken-sublattice-symmetry and the permutationally-symmetric-sublattices phases is calculated in the two approximations and turns out to be smaller in the (more accurate) star-cube approximation than in the cube one.Comment: 4 pages REVTeX + 2 PostScript figures, to be published in Phys. Rev. E as a Rapid Communicatio