Chayes L., Machta J., Redner O.
Graphical Representations for Ising Systems in External Fields
(45K, amstex)

ABSTRACT.  A graphical representation based on duplication is developed that is
suitable for the study of Ising systems in external fields. Two independent
replicas of the Ising system in the same field are treated as a single
four-state (Ashkin-Teller) model.  Bonds in the graphical representation
connect the Ashkin-Teller spins. For ferromagnetic systems it is proved
that ordering is characterized by percolation in this representation.  The
representation leads immediately to cluster algorithms; some applications
along these lines are discussed.