Antiferromagnetic Potts Models on the Square Lattice

We study the antiferromagnetic $q$-state Potts model on the square lattice for $q=3$ and $q=4$, using the Wang-Swendsen-Koteck\'y Monte Carlo algorithm and a new finite-size-scaling extrapolation method. For $q=3$ we obtain good control up to correlation length $\xi \sim 5000$; the data are consistent with $\xi(\beta) = A e^{2\beta} \beta (1 + a_1 e^{-\beta} + \ldots)$ as $\beta\to\infty$. For $q=4$ the model is disordered even at zero temperature.