Vortex dynamics and entropic forces in antiferromagnets and antiferromagnetic Potts models

It is well known in models with an interface representation, such as the dimer model, the triangular Ising antiferromagnet, the six-vertex ice model, and the three-state antiferromagnetic Potts model on the square lattice, that topological defects of opposite charge are attracted with an entropically-driven Coulomb force. We examine the Potts model in detail, show explicitly how this force is felt through local fields, and calculate the defects' mobility. We then take two approaches to measuring this force numerically. First, we quench a random initial state to zero temperature and measure the density of defects rho(t) as a function of time. While this gives some evidence for a local force, we compare it with a free diffusion experiment, and show that the asymptotic decay of rho(t) depends on the initial distribution of defects rather than the forces between them. Second, we set up initial conditions with a single pair of vortices, and measure the force between them as a function of distance. This gives reasonable agreement with theory, although finite-size effects and a lack of ergodicity play a significant role.