Weak versus strong universality in the two-dimensional random-bond Ising ferromagnet

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^av, are computed) by transfer-matrix methods. An ansatz for the size dependence of logarithmic corrections to ξ^av is proposed. Data for both random-bond and site-diluted systems show that pure system behavior (with ν=1) is recovered if these corrections are incorporated, discarding the weak-universality scenario.