Agreement Percolation and Phase Coexistence in Some Gibbs Systems

We extend some relations between percolation and the dependence of Gibbs states on boundary conditions known for Ising ferromagnets to other systems and investigate their general validity: percolation is defined in terms of the agreement of a configuration with one of the ground states of the system. This extension is studied via examples and counterexamples, including the antiferromagnetic Ising and hard-core models on bipartite lattices, Potts models, and many-layered Ising and continuum Widom-Rowlinson models. In particular our results on the hard square lattice model make rigorous observations made by Hu and Mak on the basis of computer simulations. Moreover, we observe that the (naturally defined) clusters of the Widom-Rowlinson model play (for the WR model itself) the same role that the clusters of the FortuinKasteleyn measure play for the ferromagnetic Potts models. The phase transition and percolation in this system can be mapped into the corresponding liquid-vapor transition of a one-component fluid.