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Progress of Theoretical Physics Supplement

Skeletonization, Fluctuating Mean-Field Approximations and Coherent Anomalies in Critical Phenomena

Masuo Suzuki

Published Jan 1, 1986• 22 citations
Publisher

Abstract

Concepts of skeleton order parameters and their conjugate symmetry breaking skeleton fields are introduced in order to describe the phase transition of the relevant system.It is shown that the "skeletonization" of Hamiltonians is very useful in understanding the essence of phase transitions.Singularities of skeleton susceptibilities are studied in general.There are given some examples in spin systems.Fluctuating mean-field approximations are introduced to study phase transitions qualitatively, namely to find more accurate values of critical points even in frustrated spin systems.This idea gives the basis of the coherent anomaly method (CAM).This new method is explained briefly to obtain non-classical critical exponents in cluster mean-field approximations on the basis of the concept of coherent anomalies.§ 1. Introduction

DOI
10.1143/ptps.87.1

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Journal of the Physical Society of Japan•Sep 1, 1987•73 citations
Concepts of skeleton order parameters and their conjugate symmetry breaking skeleton fields are introduced in order to describe the phase transition of the relevant system.It is shown that the "skeletonization" of Hamiltonians is very useful in understanding the essence of phase transitions.Singularities of skeleton susceptibilities are studied in general.There are given some examples in spin systems.Fluctuating mean-field approximations are introduced to study phase transitions qualitatively, namely to find more accurate values of critical points even in frustrated spin systems.This idea gives the basis of the coherent anomaly method (CAM).This new method is explained briefly to obtain non-classical critical exponents in cluster mean-field approximations on the basis of the concept of coherent anomalies.§ 1. Introduction
A new scheme of the coherent-anomaly method (CAM) is proposed to study critical phenomena in the models for which a mean-field description gives spurious first-order phase transition. A canonical series of mean-field-type approximations are constructed so that the spurious discontinuity should vanish asymptotically as the approximate critical temperature approachs the true value. The true value of the critical exponents β and γ are related to the coherent-anomaly exponents defined among the classical approximations. The formulation is demonstrated in the two-dimensional q -state Potts models for q =3 and 4. The result shows that the present method enables us to estimate the critical exponents with high accuracy by using the date of the cluster-mean-field approximations.
A cluster-effective-field theory, called the double-cluster approximation, is formulated for quantum spin systems at T=0. Combining this approximation with the coherent-anomaly method (CAM), critical phenomena in such systems can be analysed. To test their theory, the authors have estimated the critical points and critical exponents of the one-dimensional S=1/2 transverse Ising model and the one-dimensional S=1/2 XZ model. The estimates are in good agreement with the exact solutions. These results show that the CAM approach is useful in treating zero-temperature phase transitions in quantum spin systems.
A coherent anomaly method (CAM) is formulated for critical phenomena on the basis of the general idea of CAM for cooperative phenomena proposed by Suzuki. The coherent anomalies of response functions are derived phenomenologically using Kubo's linear response theory and Fisher's scaling form of correlation functions. This yields the relation between the coherent anomaly exponents and non-classical critical exponents. The convergence of the CAM critical points is proven rigorously. Some explicit applications of the CAM theory based on systematic cluster-mean-field approximations and on the mean-field transfer-matrix method are presented to show how useful the CAM is in studying critical phenomena. The CAM theory of critical dynamics is also formulated to give the dynamical critical exponent Δ ≃1.84 in the two-dimensional kinetic Ising model.
A series of spin-cluster approximation which is a mean field type approximation is derived systematically on the basis of a spin-cluster expansion formula of the free energy. Mean field critical temperature and mean field critical coefficients are calculated by the spin-cluster approximation for the two-, three- and more than four-dimensional ferromagnetic Ising systems. Analyzing them by the coherent anomaly method, the critical temperature and the critical exponents α, β, γ and δ are estimated. Accuracy of estimated values in this method are comparable to those given by other methods such as the high temperature series analysis. The present theory provides a new and useful tool to study the critical phenomena, especially of three-dimensional systems.
A general method is proposed to study non-classical scaling behaviors of cooperative systems. This is based on the appearance of a "coherent anomaly" in mean-field-type or classical approximations of cooperative systems. The degree of approximation is analytically continued to construct a parameterized expression of a whole set of approximations, using Kubo's linear response theory. General relations between scaling (or critical) exponents and "coherent-anomaly exponents" are derived using the theory of envelope and are also obtained from a finite-degree-of-approximation scaling, which is inspired by Fisher's finite-size scaling. The present coherent-anomaly method (CAM) is promising for calculating analytically non-classical scaling exponents in cooperative phenomena. A general scheme of CAM in critical phenomena is presented. It is shown that even the combination of the Weiss and Bethe approximations gives rather good estimates of static and dynamic critical exponents.
A systematic procedure of constructing a series of cluster-mean-field approximations is established to investigate the coherent anomalies of the mean-field critical coefficients \barχ( L , T c ( L )) in the two- and three-dimensional Ising models. The values of critical exponents as well as the critical temperatures are estimated by means of the new method (Coherent Anomaly Method) as T c * =2.230(0.038) J / k B , γ=1.785(0.116) for the two-dimensional square lattice and T c * =4.529(0.058) J / k B , γ=1.258(0.068) for the three-dimensional simple cubic lattice. The results suggest that the present approach to study critical phenomena is very promising.