On weak vs. strong universality in the two-dimensional random-bond Ising ferromagnet

Abstract

We address the issue of universality in two-dimensional disordered Ising systems, by considering long, finite-width strips of ferromagnetic Ising spins with randomly distributed couplings. We calculate the free energy and spin-spin correlation functions (from which averaged correlation lengths, $ξ^{ave}$, are computed) by transfer-matrix methods. An {\it ansatz} for the size-dependence of logarithmic corrections to $ξ^{ave}$ is proposed. Data for both random-bond and site-diluted systems show that pure system behaviour (with $ν=1$) is recovered if these corrections are incorporated, discarding the weak--universality scenario.