Strong-Disorder Paramagnetic-Ferromagnetic Fixed Point in the Square-Lattice +/- J Ising Model

We consider the random-bond +/- J Ising model on a square lattice as a function of the temperature T and of the disorder parameter p (p=1 corresponds to the pure Ising model). We investigate the critical behavior along the paramagnetic-ferromagnetic transition line at low temperatures, below the temperature of the multicritical Nishimori point at T (*)=0.9527(1), p (*)=0.89083(3). We present finite-size scaling analyses of Monte Carlo results at two temperature values, Ta parts per thousand 0.645 and T=0.5. The results show that the paramagnetic-ferromagnetic transition line is reentrant for T < T (*), that the transitions are continuous and controlled by a strong-disorder fixed point with critical exponents nu=1.50(4), eta=0.128(8), and beta=0.095(5). This fixed point is definitely different from the Ising fixed point controlling the paramagnetic-ferromagnetic transitions for T > T (*). Our results for the critical exponents are consistent with the hyperscaling relation 2 beta/nu-eta=d-2=0.