CRITICAL-BEHAVIOR OF THE 3-DIMENSIONAL RANDOM-FIELD ISING-MODEL - 2-EXPONENT SCALING AND DISCONTINUOUS TRANSITION

In extensive Monte Carlo simulations the phase transition of the random-field Ising model in three dimensions is investigated. The values of the critical exponents are determined via finite-size scaling. For a Gaussian distribution of the random fields it is xi diverges with an exponent nu = 1.1 +/- 0.2 at the critical temperature and that chi similar to xi(2-eta) with eta = 0.50 +/- 0.05 for the connected susceptibility and chi(dis) similar to xi(4-eta) with eta = 1.03 +/- 0.05 for the disconnected susceptibility. Together with the amplitude ratio A = lim(T) (-->) (Tc), chi(dis)/chi(2)(h(r)/T)(2) being close to one this gives further support for a two-exponent scaling scenario implying eta = 2 eta. The magnetization behaves discontinuously at the transition, i.e., beta = 0. However, no divergence for the specific heat and in particular no latent heat is found. Also the probability distribution of the magnetization does not show a multipeak structure that would be characteristic for the phase coexistence at first-order phase-transition points.