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Physical review. B, Condensed matter

Spin-spin correlations in finite systems with O(n) symmetry: Scaling hypothesis and corrections to bulk behavior

Surjit Singh, Raj K. Pathria

Published Sep 1, 1987• 33 citations
Publisher

Abstract

A scaling hypothesis is set up for the correlation function G(R,T;L) of a finite-sized system, with O(n) symmetry (n\ensuremath{\ge}2), confined to geometry ${L}^{d\mathrm{\ensuremath{-}}d\mathcal{'}}$\ifmmode\times\else\texttimes\fi{}${\ensuremath{\infty}}^{d\mathcal{'}}$ (2d4, d'\ensuremath{\le}2) and subjected to periodic boundary conditions. Finite-size effects in the correlation length \ensuremath{\xi}(T;L) are predicted in the region of first-order phase transition (T${T}_{c}$) as well as the region of second-order phase transition (T\ensuremath{\simeq}${T}_{c}$); for the correlation function, the predictions relate mostly to the regime T${T}_{c}$. To test these predictions, a detailed analytical study is carried out in the case of the spherical model and all predictions are seen to be fully borne out. In the process we find that, though a single correlation length \ensuremath{\xi} governs the scaling of the variable R in all directions, the functional dependence of the correlation function on the scaled parameter R/\ensuremath{\xi} is highly anisotropic, in that it depends very differently on the components, ${\mathrm{R}}_{\ensuremath{\perp}}$ and ${\mathrm{R}}_{?}$, of R pertaining to the finite and the infinite dimensions, respectively. Most importantly, while long-range order at temperatures below ${T}_{c}$ prevails all the way in the direction of ${\mathrm{R}}_{\ensuremath{\perp}}$ it is severely limited in the direction of ${\mathrm{R}}_{?}$. In fact, for ${R}_{?}$\ensuremath{\gg}L, the qualitative features of the correlation function become characteristic of a d'-dimensional bulk system, though \ensuremath{\xi} still pertains to the actual, finite-sized system; the net result is that long-range order in the direction of ${\mathrm{R}}_{?}$ persists only to distances small in comparison with \ensuremath{\xi} which, for d'2, is found to be O(L(L/a${)}^{(d\mathrm{\ensuremath{-}}2)/(2\mathrm{\ensuremath{-}}d\mathcal{'})}$), a being the lattice constant.

DOI
10.1103/physrevb.36.3769

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Canadian Journal of Physics•Oct 1, 1989•13 citations
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Finite-size effects in the correlation function G(R,T;L) of a spherical-model ferromagnet, confined to geometry ${\mathrm{L}}^{\mathrm{d}\mathrm{\ensuremath{-}}\mathrm{d}\ensuremath{'}}$\ifmmode\times\else\texttimes\fi{}${\mathrm{\ensuremath{\infty}}}^{\mathrm{d}\ensuremath{'}}$ (2d4,d\ensuremath{'}\ensuremath{\leqslant}2) and subjected to twisted boundary conditions, are analyzed. Focusing our attention on the region of first-order phase transition (TTc{\mathrm{T}}_{\mathrm{c}}Tc​), we examine the influence of the twist parameter \ensuremath{\tau} on the function G(R,T;L) in different regimes of the distance parameter \ensuremath{\varepsilon} (=R/L). We find that the functional dependence of the function G on the parameter \ensuremath{\varepsilon} is highly anisotropic, in that it depends very differently on the components, ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\perp}}}$ and ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\parallel}}}$, of \ensuremath{\varepsilon} pertaining to the finite and the infinite directions, respectively. While the long-range order in the direction of ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\perp}}}$ is significantly altered by the parameter \ensuremath{\tau}, it is curtailed very seriously in the direction of ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\parallel}}}$. In fact, for ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\parallel}}}$\ensuremath{\gg}1, the qualitative features of the correlation function acquire characteristics of a d\ensuremath{'}-dimensional bulk system, though the correlation length \ensuremath{\xi} appearing therein still pertains to the actual, finite-sized system. The net result is that long-range order in the direction of ${\mathbf{R}}_{\mathrm{\ensuremath{\parallel}}}$ persists only to distances small in comparison with \ensuremath{\xi} which, for d\ensuremath{'}2, is known to be O{L(L/a${)}^{(\mathrm{d}\mathrm{\ensuremath{-}}2)/(2\mathrm{\ensuremath{-}}\mathrm{d}\ensuremath{'})}$}, a being the lattice constant.
A scaling hypothesis is set up for the magnetization m(T,H;L), susceptibility \ensuremath{\chi}(T,H;L), and correlation length \ensuremath{\xi}(T,H;L) of a finite-sized system, with O(n) symmetry (n\ensuremath{\ge}2) and long-range interactions decaying as 1/${r}^{d+\ensuremath{\sigma}}$ (0<\ensuremath{\sigma}<2), confined to geometry ${L}^{d\mathrm{\ensuremath{-}}d\mathcal{'}}$\ifmmode\times\else\texttimes\fi{}${\ensuremath{\infty}}^{d\mathcal{'}}$ (\ensuremath{\sigma}<d<2\ensuremath{\sigma},d'\ensuremath{\le}\ensuremath{\sigma}) and subjected to periodic boundary conditions. Finite-size effects are predicted, in the region of a first-order phase transition (TTc{T}_{c}Tc​) as well as in the region of a second-order phase transition (T\ensuremath{\simeq}Tc{T}_{c}Tc​), for different regimes of the parameters H and L. To test these predictions, a detailed analytical study is carried out in the case of the spherical model of ferromagnetism (n=\ensuremath{\infty}), and all predictions based on the scaling hypothesis are seen to be fully borne out. In situations where L\ensuremath{\gg}\ensuremath{\xi}, finite-size corrections to standard bulk values of the various physical quantities pertaining to these models are found to vary as (\ensuremath{\xi}/L${)}^{d+\ensuremath{\sigma}}$, rather than as ${e}^{\mathrm{\ensuremath{-}}L/\ensuremath{\xi}}$, which is characteristic of models with short-range interactions.
We study the correlation function G(R,T;L) and the correlation length \ensuremath{\xi}(T;L) in a finite spherical model if size ${L}^{d\mathrm{\ensuremath{-}}d\mathcal{'}}$\ifmmode\times\else\texttimes\fi{}${\ensuremath{\infty}}^{d\mathcal{'}}$ under periodic boundary conditions and emphasize the role of the variable L/\ensuremath{\xi}(L), rather than L/\ensuremath{\xi}(\ensuremath{\infty}), as a natural scaling variable of the system throughout the transition region, including temperatures below Tc{T}_{c}Tc​(\ensuremath{\infty}). We obtain a variety of finite-size effects, some of which may have validity for all O(n) models with n>2.
We propose a finite-size scaling hypothesis for the ``singular'' part of the free-energy density, f(s){f}^{(s)}f(s)(T,H;L), and the correlation function, G(R,T,H;L), of a system with O(n) symmetry (n\ensuremath{\ge}2), confined to geometry ${L}^{d\mathrm{\ensuremath{-}}d\mathcal{'}}$\ifmmode\times\else\texttimes\fi{}${\ensuremath{\infty}}^{d\mathcal{'}}$ (d>4, d'\ensuremath{\le}2) and subjected to periodic boundary conditions. On the basis of this hypothesis, we predict finite-size effects in the zero-field susceptibility, \ensuremath{\chi}(T;L), the correlation length, \ensuremath{\xi}(T;L), and the ``singular'' part of the specific heat, c(s){c}^{(s)}c(s)(T;L), in the region of first-order phase transition (TTc{T}_{c}Tc​) as well as the region of second-order phase transition (T\ensuremath{\simeq}Tc{T}_{c}Tc​). It turns out that in both these regions the system in zero field can be described by a single, unifying variable v\ensuremath{\sim}${L}^{2(d\mathrm{\ensuremath{-}}d\mathcal{'})/(4\mathrm{\ensuremath{-}}d\mathcal{'})}$t\ifmmode \tilde{}\else \~{}\fi{}, where t\ifmmode \tilde{}\else \~{}\fi{} is an appropriate temperature parameter of the system. To test the various predictions, a detailed analytical study is carried out in the case of the spherical model of ferromagnetism (n=\ensuremath{\infty}), and all predictions are seen to be fully borne out. For general n, we compare our findings with those of the previous authors, wherever possible, and in each case the comparison turns out to be perfect.
A scaling hypothesis is set up for the magnetization m(T,H;L) and susceptibility \ensuremath{\chi}(T,H;L) of a finite-sized system, with O(n) symmetry (n\ensuremath{\ge}2), confined to geometry ${\mathit{L}}^{\mathit{d}\mathrm{\ensuremath{-}}\mathit{d}\mathcal{'}}$\ifmmode\times\else\texttimes\fi{}${\mathrm{\ensuremath{\infty}}}^{\mathit{d}\mathcal{'}}$(2d4,d'\ensuremath{\le}2) and subjected to periodic boundary conditions. Finite-size effects in both m and \ensuremath{\chi} are predicted, in the region of the first-order phase transition (TTc{T}_{c}Tc​) as well as in the region of the second-order phase transition (T\ensuremath{\simeq}Tc{T}_{c}Tc​), for different regimes of the parameters H and L. To test these predictions, a detailed analytical study is carried out in the case of the spherical model of ferromagnetism (n=\ensuremath{\infty}), and all predictions based on the scaling hypothesis are seen to be fully borne out.
The finite-size scaling hypothesis of Privman and Fisher (Phys. Rev. B, 30, 322 (1984)) is applied to systems with O(n) symmetry [Formula: see text], confined to geometry Ld − d′ × ∞d′ (where d and d′ are continuous variables such that 2 < d′ < d < 4) and subjected to periodic boundary conditions. Predictions, involving amplitudes as well as exponents, are made on the singular part of the specific heat c(s), spontaneous magnetization m0, magnetic susceptibility χ0, and correlation length ξ0 in the region of the second-order phase transition [Formula: see text]. Analytical verification of these predictions is carried out in the case of the spherical model of ferromagnetism (n = ∞), which includes evaluation of the shift in the critical temperature of the system.