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Kritische spezifische Wärme in begrenzten Systemen mit Dirichlet-Oberflächen

Ulf Mohr, V. Dohm

Published Jan 1, 2000• 0 citations
OpenAlex

Abstract

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.

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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.
A detailed analysis of the finite–size effects on the bulk critical behaviour of the d–dimensional mean spherical model confined to a film geometry with finite thickness L is reported. Along the finite direction different boundary conditions are applied: periodic (p), antiperiodic (a) and free surfaces with Dirichlet (D), Neumann (N) and a combination of Neumann and Dirichlet (ND) on both surfaces. A systematic method for the evaluation of the finite–size corrections to the free energy for the different types of boundary conditions is proposed. The free energy density and the equation for the spherical field are computed for arbitrary d. It is found, for 2 < d < 4, that the singular part of the free energy has the required finite–size scaling form at the bulk critical temperature only for (p) and (a). For the remaining boundary conditions the standard finite–size scaling hypothesis is not valid. At d = 3, the critical amplitude (related to the so called Casimir amplitude) is estimated. We obtain
We study the special point in the phase diagram of a semi-infinite system where the bulk transition is in the three-dimensional Ising universality class. To this end we performed a finite-size-scaling study of the improved Blume-Capel model on the simple cubic lattice with two different types of surface interaction. In order to check for the effect of leading bulk corrections we also simulated the spin-1/2 Ising model on the simple cubic lattice. We accurately estimated the surface enhancement coupling at the special point of these models. We find yt1=0.718(2){y}_{{t}_{1}}=0.718(2)yt1​​=0.718(2) and yh1=1.6465(6){y}_{{h}_{1}}=1.6465(6)yh1​​=1.6465(6) for the surface renormalization group exponents of the special transition. These results are compared with previous ones obtained by using field-theoretic methods and Monte Carlo simulations of the spin-1/2 Ising model. Furthermore, we study the behavior of the surface transition near the special point, and finally we discuss films with special boundary conditions at one surface and fixed ones at the other.
The recent observation by Hasan and Goble of a discrepancy between their numerical results on the specific heat of an ideal Bose gas confined to a thin-film geometry and the earlier analytic calculations of Pathria employing Neumann boundary conditions is shown to be unjustified. In those cases where a true comparison is possible, e.g., under Dirichlet or periodic boundary conditions, the observed deviations turn out to be insignificant if one uses the more recent second-order calculations of Greenspoon and Pathria rather than the original first-order ones of Pathria. For comparison under Neumann boundary conditions, fresh numerical calculations are required. We also present here second-order analytic calculations of the specific heat of this system (i) under mixed boundary conditions (Dirichlet on one wall, Neumann on the other) and (ii) under antiperiodic boundary conditions, and compare them with the relevant numerical calculations of Hasan and Goble.
A detailed analysis of the finite-size effects on the bulk critical behaviour of the d -dimensional mean spherical model confined to a film geometry with finite thickness L is reported. Along the finite direction different kinds of boundary conditions are applied: periodic ( p ), antiperiodic ( a ) and free surfaces with Dirichlet ( D ), Neumann ( N ) and a combination of Neumann and Dirichlet ( ND ) on both surfaces. A systematic method for the evaluation of the finite-size corrections to the free energy for the different types of boundary conditions is proposed. The free energy density and the equation for the spherical field are computed for arbitrary d . It is found, for 2 < d < 4, that the singular part of the free energy has the required finite-size scaling form at the bulk critical temperature only for ( p ) and ( a ). For the remaining boundary conditions the standard finite-size scaling hypothesis is not valid. At d = 3, the critical amplitude of the singular part of the free energy (related to the so-called Casimir amplitude) is estimated. We obtain Δ ( p ) = −2ζ(3)/(5π) = −0.153 051..., Δ ( a ) = 0.274 543... and Δ ( ND ) = 0.019 22..., implying a fluctuation-induced attraction between the surfaces for ( p ) and repulsion in the other two cases. For ( D ) and ( N ) we find a logarithmic dependence on L .
One of the most striking aspects of critical behaviour is that of the crucial role played by the geometry of the System. Critical exponents depend in a non-trivial manner on the dimensionality d . The very existence of a phase transition depends on the way in which the infinite volume limit is taken, as discussed in Section 4.4. This happens because the critical fluctuations, which determine the universal properties, occur at long wavelengths and are therefore very sensitive to the large scale geometry. By contrast, the non-critical properties of a system are sensitive to fluctuations on the scale of the correlation length and are therefore much less influenced. This line of reasoning also suggests that not all points in a system are equivalent in the way the local degrees of freedom couple to these critical fluctuations. So far, we have considered the behaviour of scaling operators only at points deep inside the bulk of a system. Near a boundary, however, the local environment of a given degree of freedom is different, and we might expect to find different critical properties there. In general, such differences should extend into the bulk only over distances of the order of the bulk correlation length. However, at a continuous bulk phase transition, this distance diverges, and we should expect the influence of boundaries to be more pronounced. The simplest modification of the bulk geometry to consider is that of a ( d − 1)-dimensional hyperplane bounding a semi-infinite d -dimensional system.