Abstract
A uniqueness condition for Gibbs measures is given. This condition is stated in terms of (absence of) a certain type of percolation involving two independent realisations. This result can be applied in certain concrete situations by comparison with “ordinary” percolation. In this way we prove that the Ising antiferromagnet on a square lattice has a unique Gibbs measure if β(4−|h⥻)<1/2ln(P c /(1−P c )), whereh denotes the external magnetic field, β the inverse temperature, andP c the critical probability for site percolation on that lattice. SinceP c is larger than 1/2, this extends a result by Dobrushin, Kolafa and Shlosman (whose proof was computer-assisted).
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