Spontaneous Magnetization in the Disorder dominated Phase of the Twodimensional Random Bond Ising Model

Abstract

The selfconsistent approach to the 2D Ising Model with quenched random bonds is extended to the full lattice theory of four real fermions. The additional degrees of freedom, neglected in the renormalization-group theory, lead to a new phase between the ferromagnetic and the paramagnetic phase. The disorder averaged spin-spin correlation function decays exponentially with distance. The corresponding correlation length is $1/\eta^2$, where $\eta$ denotes the order parameter of the new phase introduced by Ziegler.