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Journal of Physics A Mathematical and General

Universal order-parameter profiles in confined critical systems with boundary fields

Theodore W. Burkhardt, Erich Eisenriegler

Published Feb 1, 1985• 69 citations
Publisher

Abstract

The authors consider systems at the bulk critical temperature with an ordering field applied at the boundary. For various boundary geometries, universal expressions for the spatial dependence of the order parameter are obtained by conformal mapping of the known results for the half space. Analytical results for strip, wedge and other two-dimensional geometries and for systems with spherical boundaries in arbitrary dimension are given.

DOI
10.1088/0305-4470/18/2/006

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•Jan 1, 2000•0 citations
The authors consider systems at the bulk critical temperature with an ordering field applied at the boundary. For various boundary geometries, universal expressions for the spatial dependence of the order parameter are obtained by conformal mapping of the known results for the half space. Analytical results for strip, wedge and other two-dimensional geometries and for systems with spherical boundaries in arbitrary dimension are given.
Conformal transformations can be used to obtain the order parameter for two-dimensional systems at criticality in finite geometries with fixed boundary conditions on a connected boundary. To the known examples of this class (such as the disk and the infinite strip) we contribute the case of a rectangle. We show that the order parameter profile for simply connected boundaries can be represented as a universal function (independent of the criticality model) raised to the power $\frac{1}{2}\ensuremath{\eta}.$ The universal function can be determined from the Gaussian model or equivalently a problem in two-dimensional electrostatics. We show that fitting the order parameter profile to the theoretical form gives an accurate route to the determination of $\ensuremath{\eta}.$ We perform numerical simulations for the Ising model and percolation for comparison with these analytic predictions, and apply this approach to the study of the planar rotor model.
Scaling density profiles of critical O(N)-symmetric systems confined to a film geometry are analyzed by field-theoretic renormalization-group techniques in d=4-\ensuremath{\varepsilon} spatial dimensions. At bulk criticality we explicitly calculate the energy density profile for five different boundary conditions and determine their corresponding universal scaling functions. In order to discuss the influence of the boundary conditions at the two surfaces of the film on the global shape of the profiles, a suitable exponentiation scheme is constructed. Using these exponentiated scaling functions as well as results from conformal field theory, the shapes of the energy density profiles in d=2,3, and 4 are compared. Finally, we discuss the short-distance behavior of the energy density profiles close to the surfaces of the film as it follows from the knowledge of the full profiles in first order \ensuremath{\varepsilon}.
Off-diagonal profiles of local densities (e.g. order parameter or energy density) are calculated at the bulk critical point, by conformal methods, on a strip with transverse coordinate v, for different types of boundary conditions (free, fixed and mixed). Such profiles, which are defined by the non-vanishing matrix element of the appropriate operator between the ground state and the corresponding lowest excited state of the strip Hamiltonian, enter into the expression of two-point correlation functions on a strip. They are of interest in the finite-size scaling study of bulk and surface critical behaviour since they allow the elimination of regular contributions. The conformal profiles, which are obtained through a conformal transformation of the correlation functions from the half-plane to the strip, are in agreement with the results of a direct calculation, for the energy density of the two-dimensional Ising model.
The conformal-invariance approach is extended to two-dimensional critical systems with mixed ab boundary conditions. The profiles 〈\ensuremath{\varphi}(r)〉ab{\mathrm{〉}}_{\mathit{a}\mathit{b}}〉ab​ of the magnetization and the energy density in Ising and Q-state Potts strips with +- and fixed-free boundary conditions are derived. The profile of an arbritrary operator degenerate at level 2 is expressed in terms of hypergeometric functions. The universal amplitudes Aab{\mathit{A}}_{\mathit{a}\mathit{b}}Aab​Bab{\mathit{B}}_{\mathit{a}\mathit{b}}Bab​ of the Casimir force and the Fisher--de Gennes distant-wall correction to the profile satisfy Bab{\mathit{B}}_{\mathit{a}\mathit{b}}Bab​=-4\ensuremath{\pi}${\mathit{x}}_{\mathrm{\ensuremath{\varphi}}}$${\mathit{c}}^{\mathrm{\ensuremath{-}}1}$Aab{\mathit{A}}_{\mathit{a}\mathit{b}}Aab​. Here ${\mathit{x}}_{\mathrm{\ensuremath{\varphi}}}$ is the scaling dimension of \ensuremath{\varphi}(r) and c is the central charge.
The present work investigates the influence of real boundary conditions on the critical behaviour of confined system close to the temperature of a second order phase transition of the bulk. This is done analytically by applying the methods of the field theory of critical phenomena to a half-space geometry with a completely suppressed order parameter on the surface, the so-called Dirichlet boundary conditions. For the first time the surface specific heat is calculated to next to leading order in a fixed dimension both above and below the critical temperature. The results are compared with the data of recent experiments on Helium in plate geometry close to the lambda transition. Apart from the experimentally known bulk amplitudes no adaptable parameters are used. Above the critical temperature a semi-quantitative agreement of theory and experiment is found, whereas the experimental data cannot be explained below the lambda point. Possible reasons for this discrepancy are discussed. Secondly a new method to approximate Dirichlet boundary conditions in Monte Carlo simulations of lattice spin systems is presented. In comparison with established realizations both a better suppression of the order parameter on the boundary and the theoretically predicted scaling behaviour is achieved. Data generated with the new method shows agreement with theory within the expected errors.