Perturbative analysis of disordered Ising models close to criticality

Abstract We consider a two-dimensional Ising model with random i.i.d. nearest-neighbor ferromagnetic couplings and no external magnetic field. We show that, if the probability of supercritical couplings is small enough, the system admits a convergent cluster expansion with probability one. The associated polymers are defined on a sequence of increasing scales; in particular the convergence of the above expansion implies the infinite differentiability of the free energy but not its analyticity. The basic tool in the proof are a general theory of graded cluster expansion and a stochastic domination of the disorder. MSC2000. Primary 82B44, 60K35