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Journal of Physics A Mathematical and General

Correlation functions of the classical Heisenberg model. II. Low temperature behaviour

J Rae

Published Mar 1, 1975• 3 citations
Publisher

Abstract

For pt. I, see ibid, vol.8, no.3, 347 (1975). In the first of this pair of papers exact expressions were obtained for the pair correlation functions and susceptibilities of the classical Heisenberg model in terms of ellipsoidal wavefunctions. Here the author examines the low temperature behaviour of these quantities. This necessitates an asymptotic expression for the ellipsoidal functions. The derivation of such an expression follows the method of Langer and is to leading order only. This asymptotic form allows explicit calculation of the eigenvalues of the transfer matrix (to leading order) and hence a discussion of the low temperature behaviour of the correlation functions and susceptibilities.

DOI
10.1088/0305-4470/8/3/009

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For pt. I, see ibid, vol.8, no.3, 347 (1975). In the first of this pair of papers exact expressions were obtained for the pair correlation functions and susceptibilities of the classical Heisenberg model in terms of ellipsoidal wavefunctions. Here the author examines the low temperature behaviour of these quantities. This necessitates an asymptotic expression for the ellipsoidal functions. The derivation of such an expression follows the method of Langer and is to leading order only. This asymptotic form allows explicit calculation of the eigenvalues of the transfer matrix (to leading order) and hence a discussion of the low temperature behaviour of the correlation functions and susceptibilities.
The authors derive the thermodynamics and the static correlation functions for the classical Heisenberg chain with the anisotropies within a low-temperature steepest-descent approach. As regards the thermodynamics, they find agreement with transfer matrix and 'ideal-gas' phenomenological results. For the longitudinal correlation function the authors find an antiferromagnetic-type behaviour with a correlation length inversely proportional to the domain wall density. The transverse in-plane correlation function exhibits a damped oscillatory behaviour due to domain-wall-spin interference. The transverse out-of-plane correlations, on the other hand, are exponentially damped with the correlation length increasing as the easy-axis limit is approached.
Exact expressions for the partition function, spin pair correlation function, and susceptibility of the onedimensional isotropic classical Heisenberg model are obtained in zero external field with cyclic boundary conditions. It is shown that the methods used to derive these results enable the partition functions and susceptibilities of finite clusters of interacting classical spins to be evaluated in terms of the $3n\ensuremath{-}j$ symbols of Wigner. Exact results in one dimension are also obtained for the partition function and susceptibility of a "planar" classical Heisenberg model. In this model the spin vectors interact via a Heisenberg coupling but each spin vector is restricted to lie in a plane.The anisotropic classical Heisenberg model described by the Hamiltonian $\mathcal{H}=\ensuremath{-}\ensuremath{\Sigma}\stackrel{}{(\mathrm{ij})}2({{J}_{\mathrm{ij}}}^{x}{{s}_{i}}^{x}{{s}_{j}}^{x}+{{J}_{\mathrm{ij}}}^{y}{{s}_{i}}^{y}{{s}_{j}}^{y}+{{J}_{\mathrm{ij}}}^{z}{{s}_{i}}^{z}{{s}_{j}}^{z})\ensuremath{-}mH\ensuremath{\Sigma}\stackrel{N}{j=1}{{s}_{i}}^{z},$ where six{{s}_{i}}^{x}si​x, siy{{s}_{i}}^{y}si​y, and siz{{s}_{i}}^{z}si​z are components of the unit vector si{\mathbf{s}}_{i}si​, is also considered. A perturbation series for the zero-field free energy of the anisotropic model in one dimension with nearest-neighbor interactions Jijx=Jijy=J{{J}_{\mathrm{ij}}}^{x}={{J}_{\mathrm{ij}}}^{y}=JJij​x=Jij​y=J and ${{J}_{\mathrm{ij}}}^{z}=\ensuremath{\gamma}J$ is developed in powers of $\ensuremath{\gamma}\ensuremath{-}1$ using the isotropic model as the unperturbed system. Detailed calculations are performed to third order in $\ensuremath{\gamma}\ensuremath{-}1$. It is found that the perturbation series for the energy per spin breaks down as $T\ensuremath{\rightarrow}0$. A high-temperature series expansion for the anisotropic model, which is valid for a general interaction potential and lattice, is derived by generalizing the methods developed by Horwitz and Callen for the Ising model. This series is rearranged to give a simplified diagram expansion. Finally, a practical technique for calculating the high-temperature series expansions of the zero-field free energy and susceptibility of the isotropic classical Heisenberg model is presented.
For pt.I see ibid., vol.10, p.2869 (1977). The local single-particle and density correlation functions for the symmetric Wolff model (1961) are studied in the asymptotic weak-coupling-long-time-low-temperature limit. The scattering channels are separated by means of the Stratonovich-Hubbard transformation and the associated one-body problem is solved explicitly in the low-temperature limit. A steepest-descent argument yields expressions for the correlation functions in the weak-coupling limit. The correlation functions are characterised by a time dependent phase shift which decays on a time scale of the order of the bandwidth. In the intermediate and strong coupling regime many-body effects intervene and the calculation breaks down; in that context comments are made on some recent work by Mattis (1976).
In this part labelled II we examine the spin correlations and the susceptibility. We use a similar method which has allowed to derive a closed-form expression of the zero-field partition function ZN(0), for 2D square lattices composed of (2N+1)2classical spins isotropically coupled [1]. We rigorously show that the spin correlation vanishes in the zero-field limit, except at T=0 K. Thus, the critical temperature is TC=0 K, in agreement with Mermin-Wagner's theorem. For calculating the spin-spin correlation, we show that it is necessary to distinguish a correlation domain in which the correlation path is confined and a wing domain (Theorem 1). In the thermodynamic limit (N→+∞), we give a general closed-form expression for the spin-spin correlation between any two lattice sites. We prove that all the possible paths have the same analytic expression and correspond to the shortest ones in agreement with the classical principle of least action and its quantum version (Theorem 2). As a result and for the first time, we derive the closed-form expression for the susceptibility, without any approximation. We finally test previous experimental fits and we show that the use of a truncated expansion for the susceptibility was totally justified.