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arXiv:cond-mat/0201147 · Physical review. B, Condensed matter

Quality of the mean-field approximation: A low-order generalization yielding realistic critical indices for three-dimensional Ising-class systems

Alexey N. Rubtsov

Published Aug 29, 2002• 4 citations
arXivHTMLPDF

Abstract

Modification of the renormalization-group approach, invoking Stratonovich transformation at each step, is proposed to describe phase transitions in three-dimensional Ising-class systems. The proposed method is closely related to the mean-field approximation. The low-order scheme that works well for a wide thermal range is consistent with a scaling hypothesis and predicts very reasonable values of critical indices.

DOI
10.1103/physrevb.66.052107

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Modification of the renormalization-group approach, invoking Stratonovich transformation at each step, is proposed to describe phase transitions in three-dimensional Ising-class systems. The proposed method is closely related to the mean-field approximation. The low-order scheme that works well for a wide thermal range is consistent with a scaling hypothesis and predicts very reasonable values of critical indices.
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The Niemeijer-Van Leeuwen cumulant expansion is used in the renormalization-group analysis of the critical-point behavior of the Ising model. Second- and third-order results are obtained for the simple cubic and square lattices, respectively, for a variety of cell sizes, using a parameter-dependent renormalization transformation introduced by Kadanoff and Houghton. Although the results in two dimensions are in general good, there is no evidence of any convergence, with the best values for the critical exponents and critical temperature being given by the second-order theory. In three dimensions the values found for the critical temperature and magnetic scaling index for a cell of 27 spins are reasonable, but the corresponding thermal scaling index is quite poor.
The mean-field approximation is used for the evaluation of the Niemeijer---Van Leeuwen real-space renormalization. Although this new method is very simple, it gives reasonable results for the critical properties of two-, three-, and four-dimensional Ising models, for the transition temperature Tc{T}_{c}Tc​ of an Ising model with clusters and for the two-dimensional Ising antiferromagnet in an external magnetic field. Using this new mean-field renormalization in connection with the real-space renormalization of the kinetic Ising model, which recently has been suggested by the author, we obtain the long-time relaxation of the magnetization for all temperatures above Tc{T}_{c}Tc​. In the last application the critical properties of nnn-dimensional spins in two and three space dimensions are calculated for arbitrary nnn. Here the mean-field approximation and Monte Carlo methods are used to evaluate the renormalization step.
These lectures describe the physics of phase transitions and critical phenomena. After introducing the thermodynamics of phase transitions, these general considerations are made explicit by presenting statistical mechanical calculations on a lattice gas model in the mean-field approximation. Then the Landau theory of phase transitions is presented. The form of free energy in the Landau theory is motivated by an explicit mean field calculation for the Ising model. This is followed by a discussion of spatial correlations and the breakdown of the mean field theory due to increase of spatial fluctuations near the critical point. This calculation also brings out the role of dimension of the system for the critical behaviour. The basic ideas of Renormalization Group (RG) are then introduced and illustrated by a simple RG calculation for the one-dimensional Ising model. Real-space RG calculations are then applied to the two-dimensional Ising model on the triangular lattice, which, in fact, brings forth several key features of the RG technique. A discussion of more general aspects of the RG formalism, such as scaling for spatial correlation functions, relevant and irrelevant couplings, universality, upper critical dimension, etc. are introduced.
A new method of correlation reduction near the critical temperature is introduced for a classical spin system. This method is based on an assumption much less restrictive than the usual mean-field approximation. It permits an important parameter to be determined realistically. The transition temperature of the Ising ferromagnet is expressed very simply in a closed form. The values of the transition temperature evaluated with various approximations in this method are presented for three-dimensional Ising lattices.