Signed FK percolation in Ising spin glasses
We introduce signed Fortuin-Kasteleyn (FK) percolation for the +/-J random-interaction Ising model to characterize nonzero uniform magnetization density and two-replica overlap within the class of Gibbs distributions whose infinite-volume Edwards-Sokal (ES)/FK representation has at most one infinite FK cluster almost surely. The construction refines ordinary FK connectivity by retaining the parity of coupling products along open paths. Using the infinite-volume ES representation and its DLR equations, we study pairs of companion objects and the compatible spin assignments on them. In one replica, companions are defined by positive-parity FK connections. For extremal states satisfying the infinite-cluster assumption and almost-sure existence of companion densities along the chosen van Hove sequence, nonzero limiting expected magnetization density is equivalent to density distinguishability of the companions of the infinite FK cluster. For two independent replicas, companions are defined by FK connections coherent in parity. For products of extremal one-replica states satisfying the infinite-cluster assumption and almost-sure existence of the companion densities along the chosen sequence, density distinguishability of the companions is equivalent to nonzero limiting expected mutual overlap. For two independent replicas drawn from the same extremal state, this specializes to the Edwards-Anderson self-overlap.