  At zero temperature, the 3-state antiferromagnetic Potts model on a square
lattice maps exactly onto a point of the 6-vertex model whose long-distance
behavior is equivalent to that of a free scalar boson. We point out that at
nonzero temperature there are two distinct types of excitation: vortices, which
are relevant with renormalization-group eigenvalue 1/2; and non-vortex
unsatisfied bonds, which are strictly marginal and serve only to renormalize
the stiffness coefficient of the underlying free boson. Together these
excitations lead to an unusual form for the corrections to scaling: for
example, the correlation length diverges as \beta \equiv J/kT \to \infty
according to \xi \sim A e^{2\beta} (1 + b\beta e^{-\beta} + ...), where b is a
nonuniversal constant that may nevertheless be determined independently. A
similar result holds for the staggered susceptibility. These results are shown
to be consistent with the anomalous behavior found in the Monte Carlo
simulations of Ferreira and Sokal.
