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Physical review. B, Condensed matter

Exact solutions for Ising-model odd-number correlations on planar lattices

Jeremiah H. Barry, T. Tanaka, Mahfuza Khatun, C. H. Múnera

Published Aug 1, 1991• 45 citations
Publisher

Abstract

Simple procedures are first used to obtain exact solutions of highly-localized odd-number Ising correlations on the kagom\'e, square, and honeycomb lattices. To extend these results, a systematic and unifying method is then developed and demonstrated for finding exact solutions of n-site (with n an odd integer) Ising correlations on various planar lattices. The method combines five transformation or mapping theorems and linear-algebraic correlation identities of the triangular Ising model supplemented by a foreknowledge of its spontaneous magnetization and three select triplet correlations. In particular, considering a select seven-site cluster of the triangular Ising model, the knowledge of all its eleven odd-number correlations defined upon this cluster is shown sufficient for determining exactly all honeycomb, decorated-honeycomb, and kagom\'e Ising odd-number correlations upon their correspondingly select 10-, 19-, and 9-site clusters, respectively. The direct applicability of the catenated mapping theorems and relative ease of the calculational procedures are highlighted by the resulting large numbers of multisite correlation solutions (e.g., approximately 80 and 50 for the honeycomb and kagom\'e Ising models, respectively), the large ${\mathit{n}}_{\mathrm{max}}$ values (${\mathit{n}}_{\mathrm{max}}$=9, 9, and 19, respectively, for the honeycomb, kagom\'e, and decorated-honeycomb Ising models), and convenient prescriptions for extracting critical amplitudes. The results also offer examples of correlation degeneracies and other linear-algebraic correlation identities that do not depend explicitly upon the interaction parameters.

DOI
10.1103/physrevb.44.2595

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Simple procedures are first used to obtain exact solutions of highly-localized odd-number Ising correlations on the kagom'e, square, and honeycomb lattices. To extend these results, a systematic and unifying method is then developed and demonstrated for finding exact solutions of n-site (with n an odd integer) Ising correlations on various planar lattices. The method combines five transformation or mapping theorems and linear-algebraic correlation identities of the triangular Ising model supplemented by a foreknowledge of its spontaneous magnetization and three select triplet correlations. In particular, considering a select seven-site cluster of the triangular Ising model, the knowledge of all its eleven odd-number correlations defined upon this cluster is shown sufficient for determining exactly all honeycomb, decorated-honeycomb, and kagom'e Ising odd-number correlations upon their correspondingly select 10-, 19-, and 9-site clusters, respectively. The direct applicability of the catenated mapping theorems and relative ease of the calculational procedures are highlighted by the resulting large numbers of multisite correlation solutions (e.g., approximately 80 and 50 for the honeycomb and kagom'e Ising models, respectively), the large nmax{\mathit{n}}_{\mathrm{max}}nmax​ values (nmax{\mathit{n}}_{\mathrm{max}}nmax​=9, 9, and 19, respectively, for the honeycomb, kagom'e, and decorated-honeycomb Ising models), and convenient prescriptions for extracting critical amplitudes. The results also offer examples of correlation degeneracies and other linear-algebraic correlation identities that do not depend explicitly upon the interaction parameters.
A systematic and unifying method is developed and demonstrated for obtaining exact solutions of n-site (n even integer) Ising correlations on various planar lattices. The scheme, which is exceedingly more simple than using solely traditional Pfaffian techniques, embodies five mapping theorems in alliance with algebraic correlation identities. In the theoretical framework, the triangular Ising model plays an overarching role. In particular, considering a select 7-site cluster of the triangular Ising model, the knowledge of all its 11 even-number correlations defined upon this cluster (where only four of the correlations need to be actually calculated by Pfaffian procedures) is shown to be sufficient for determining exactly all honeycomb, decorated-honeycomb, and kagom\'e Ising model even-number correlations upon their correspondingly select 10-site, 19-site, and 9-site clusters, respectively. The relative ease and direct applicability of the present approach are highlighted not only by the resulting large numbers of n-site (n even-integer) correlation solutions (e.g., approximately 85 and 50 for the honeycomb and kagom\'e Ising models, respectively) and their large nmax{n}_{\mathrm{max}}nmax​ values (nmax=8{n}_{\mathrm{max}=8}nmax=8​,10,18 for the kagom\'e, honeycomb, and decorated-honeycomb Ising models, respectively) but also by the realization that the exact solutions for Ising multisite correlations upon the kagom\'e lattice (one of the four regular lattices in two dimensions) are apparently the first to explicitly appear in the literature beyond its nearest-neighbor pair correlation (energy).
The 3-12 (or extended kagom\'e) lattice is a three-coordinated irregular planar lattice having physical applications. Viewing its sites as the decoration sites of a doubly decorated honeycomb lattice, one proves via local star-triangle and double decoration-decimation transformations that 3-12 Ising correlations can be conveniently represented as linear combinations of honeycomb Ising correlations. Existent knowledge of all honeycomb Ising correlations upon a select (spatially compact) 10-site cluster is thus sufficient to determine all 3-12 Ising correlations upon an associated 18-site cluster. The total number of 3-12 Ising correlations defined upon this 18-site cluster is exceedingly large, but their actual count is less significant than the realization that each can now be found in a systematic and efficient fashion. Examples of resulting exact solutions for both even- and odd-number multisite correlations of the 3-12 Ising ferromagnet are presented at all temperatures. A simple scaling relationship is established between the asymptotic forms of the pair correlation in the 3-12 and honeycomb Ising models. Besides providing relatively direct derivations (no explicit magnetic fields or field derivatives) for the spontaneous magnetization and internal energy of the 3-12 Ising model, the mapping methods may be repeated recursively to secure Ising multisite correlations upon various other irregular planar lattices.
To obtain an unusually large set of exact solutions for even-number localized correlations of the standard square Ising model, a method is developed that combines traditional Pfaffian techniques with linear-algebraic systems of correlation identities having interaction-dependent coefficients. Two eight-site clusters (subclusters of the ``Greek cross'' cluster) are studied, and altogether sixty different even-number correlations are determined exactly at all temperatures. Besides demonstrating the existence of degeneracies within the set of exact solutions, the solution curve for an eight-site correlation is evidently the first of such large order to be displayed for an Ising model on any regular planar lattice. Significant time as well as labor efficiency of the method is clearly demonstrated since relatively few correlations need to be actually calculated by Pfaffian techniques in order to obtain large additional numbers of multisite correlation solutions using much simpler linear-algebraic procedures.
This thesis project involved the study of exact solutions for magnetic spin correlations on a regular- and a decorated-square Ising lattice structure. Existent knowledge of correlations on a regular-square Ising structure is sufficient to determine exact correlation values on a select, spatially compact, decorated structure. These values can be represented as linear combinations of regular-square Ising spin correlation through the use of linear-algebra and the decoration-iteration transformation. Multispin localized correlations are studied over a broad range of temperatures and are found to decrease with increasing temperature. At very low temperatures, all correlation are found to saturate to unity regardless of spin separation and quantity of spins involved.
We develop a unified fermionic-field formulation of the two-dimensional Ising model on several planar lattices using the Kac-Ward representation. Grassmann fields are associated with directed lattice links, while changes in the direction of fermionic trajectories are encoded by the corresponding Kac-Ward phase factors. The resulting quadratic action reduces, after Fourier transformation, to a finite-dimensional momentum-space fermionic matrix. Its determinant determines the excitation spectrum, while its zero modes define the critical surface. We apply this construction to the square, honeycomb, triangular, kagomé, and dual kagomé (dice or T3T_3T3​) lattices. The known critical couplings of the homogeneous models are reproduced as consistency checks. For the fully anisotropic kagomé model with three independent couplings we recover the previously known fully anisotropic kagomé critical surface within the present Kac-Ward construction and the corresponding low-energy spectral equation. We construct the analogous 12×1212\times1212×12 fermionic matrix for the anisotropic dice lattice and derive its critical surface, which is related to that of the kagomé lattice by the Ising duality transformation. The same fermionic formulation also provides direct access to physical correlation functions. Expressing the spin-spin correlators through fermionic Green functions and the corresponding Toeplitz/Pfaffian representation, we obtain the spontaneous magnetization of the anisotropic kagomé model and of the six-coordinated sublattice of the anisotropic dice model. In both cases the magnetization is controlled by the same spectral polynomial that determines the fermionic gap and vanishes on the corresponding critical surface. In the homogeneous limits the known results are recovered, and the standard two-dimensional Ising magnetization exponent β=1/8β=1/8β=1/8 follows.