Chayes L, McKellar D, Winn B.
Percolation and Gibbs States Multiplicity for Ferromagnetic
Ashkin--Teller Models on $\Bbb Z^2$
(33K, amstex)
ABSTRACT. For a region of the nearest neighbor ferromagnetic Ashkin--Teller
spin systems on $\Bbb Z^2$, we characterize the existence of
multiple Gibbs states via percolation. In particular, there are multiple
Gibbs states if and only if there exists percolation of any
of the spin types. (I.e., the magnetized states are characterized
by percolation of the dominant species.) This result was previously known only
for the Potts models on $\Bbb Z^2$.