Critical exponents of the Ising model on low-dimensional fractal media

The critical behavior of the Ising model on fractal substrates with noninteger Hausdorff dimension d(H) < 2 and infinite ramification order is studied by means of the short-time critical dynamic scaling approach. Our determinations of the critical temperatures and critical exponents beta, gamma, and nu are compared to the predictions of the Wilson-Fisher expansion, the Wallace-Zia expansion, the transfer matrix method, and more recent Monte Carlo simulations using finite-size scaling analysis. We also determined the effective dimension (d(ef)), which plays the role of the Euclidean dimension in the formulation of the dynamic scaling and in the hyperscaling relationship d(ef) = 2 beta/nu + gamma/nu. Furthermore, we obtained the dynamic exponent z of the nonequilibrium correlation length and the exponent theta that governs the initial increase of the magnetization. Our results are consistent with the convergence of the lower-critical dimension towards d = 1 for fractal substrates and suggest that the Hausdorff dimension may be different from the effective dimension. (C) 2008 Elsevier B.V. All rights reserved.