Entropic Coulomb Forces in Ising and Potts Antiferromagnets and Ice Models

We study topological defects in the triangular Ising antiferromagnet, a related model on the square lattice equivalent to the six-vertex ice model, and the three-state antiferromagnetic Potts model on the square lattice. We suggest that in each case, defects are attracted or repelled with a Coulomb force mediated through correlations in the surrounding lattice. Since the energy of the two defects does not depend on the distance between them, this force is entropically driven. We give numerical evidence for such a force, both from quenches to zero temperature and direct measurements of vortex pairs. For the three-state Potts model, the strength of the force is in reasonable agreement with its theoretical value. <p> <p> Appeared in Physical Review E60 (1999) 5344-5351.