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Physical review. B, Condensed matter

Non-Gaussian approach to critical fluctuations in the Landau-Ginzburg model and finite-size scaling

Jack A. Tuszyński, A. Wierzbicki

Published Apr 1, 1991• 10 citations
Publisher

Abstract

This paper deals with applications of non-Gaussian method to treat spatial fluctuations and their manifestations in the near-critical region. The basis for calculations is the celebrated Landau-Ginzburg-Wilson Hamiltonian. The results obtained are largely analytical, but numerical computations have also been done for further support and illustration. We have calculated expressions for the partition function, dynamic susceptibility, and total scattering intensity, as well as the correlation function and the correlation range. It is demonstrated that, in the limit of the volume V\ensuremath{\rightarrow}\ensuremath{\infty} or the reduced temperature \ensuremath{\tau}==(T-${\mathit{T}}_{\mathit{c}}$)/${\mathit{T}}_{\mathit{c}}$ being large enough, the obtained results all tend to their Gaussian estimates. However, for finite volumes or small but nonzero \ensuremath{\epsilon}, asymptotic behaviors are found that are mostly governed by exponential functional relations. Two illustrative examples are provided for experimental support: (1) Brillouin scattering in ferroelastics and (2) Raman scattering in simple fluids.

DOI
10.1103/physrevb.43.8472

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Cambridge University Press eBooks•Mar 6, 2014•1 citation
This paper deals with applications of non-Gaussian method to treat spatial fluctuations and their manifestations in the near-critical region. The basis for calculations is the celebrated Landau-Ginzburg-Wilson Hamiltonian. The results obtained are largely analytical, but numerical computations have also been done for further support and illustration. We have calculated expressions for the partition function, dynamic susceptibility, and total scattering intensity, as well as the correlation function and the correlation range. It is demonstrated that, in the limit of the volume V\ensuremath{\rightarrow}\ensuremath{\infty} or the reduced temperature \ensuremath{\tau}==(T-Tc{\mathit{T}}_{\mathit{c}}Tc​)/Tc{\mathit{T}}_{\mathit{c}}Tc​ being large enough, the obtained results all tend to their Gaussian estimates. However, for finite volumes or small but nonzero \ensuremath{\epsilon}, asymptotic behaviors are found that are mostly governed by exponential functional relations. Two illustrative examples are provided for experimental support: (1) Brillouin scattering in ferroelastics and (2) Raman scattering in simple fluids.
This paper investigates the application of the renormalized Gaussian approach (RGA) on spatial fluctuations and their manifestations in the vicinity of the critical point. The basis for calculation is the Landau–Ginzburg–Wilson (LGW) Hamiltonian ( H LGW ). Within the framework of this approach, the model predictions are examined in some detail and the importance of fluctuations is established. Analytic expressions for the correlation length, susceptibility, correlation function as well as the specific heat are calculated and an approximate Ginzburg criterion is deduced. The approach also establishes the limitations of the mean-field theory description, since it is found that the mean-field critical temperature T c is a characteristic scale of temperature linked to the thermal fluctuations rather than the transition temperature. By applying the RGA, the concept of dimensionality appears essential in the manifestation and analysis of phase transitions since the approach suggests the dependence of some critical parameters and exponents on the dimensionality, as well as the quantum character. Also, it is shown that at low temperature, the correlation length of 1D systems becomes infinite at T = 0 K in accord with the Landau and Mermin–Wagner theorems. Finally, three illustrative examples are provided as analysis support in order to demonstrate the usefulness of the approach for a variety of phase transitions: (1) non-conventional superconductors, (2) localized magnetism, and (3) ferroelectricity.
This paper investigates the role of statistical fluctuations about the equilibrium value of the order parameter in the vicinity of a phase-transition temperature. The systems under consideration are assumed to be of finite size, but the behavior in the thermodynamic limit is also described. Our objective is to examine the simplest exactly solvable model of non-Gaussian static fluctuations for samples of finite size. To this end a Landau form is employed for the model Hamiltonian and various expansions are analyzed in order to describe first- and second-order phase transitions, field-induced transitions, and liquid-vapor critical phenomena. The approach differs from previous treatments primarily in the application of a quartic non-Gaussian model for statistical fluctuations which redresses some of the shortcomings of the standard Gaussian approximation. In particular, it is found that for finite values of V (volume) no singularities exist even at the critical temperature. The treatment is generalized by examining a class of non-Gaussian distribution functions, and particular emphasis is placed on strict adherence to the uniform convergence criterion so that general conclusions can be drawn.
The critical behavior of simple single-component fluids near the liquid-vapor critical point (and of other systems in the same universality class) is examined via an asymmetric spin Hamiltonian in Landau-Ginzburg-Wilson form. The leading important odd terms to be added to an Ising-type system are a five-spin interaction $\ensuremath{\int}\ensuremath{\phi}^{5}(x)$ and a nonlocal cubic interaction $\ensuremath{-}\ensuremath{\int}\ensuremath{\phi}^{2}\ensuremath{\nabla}^{2}\ensuremath{\phi}$. These two operators can be combined into two different eigenoperators with distinct physical interpretations. One combination can be shown to be exactly equivalent to the mixing of field variables (e.g., chemical potential and temperature) in the temperaturelike variable of an Ising or Ising-type system. This mixing has been used phenomenologically, found in certain models, and justified on general geometric grounds. This is the first proof that all asymmetric models, assuming universality and barring accidents, will have such mixing; this implies the universal occurrence of a ${t}^{1\ensuremath{-}a}$ fluid diameter. The second combination is treated by renormalization-group methods and its contributions to the Helmholtz free energy, magnetic equation of state, and correlation length are calculated to $O({\ensuremath{\epsilon}}^{2}), \ensuremath{\epsilon}\ensuremath{\equiv}4\ensuremath{-}d$. There are several novel features of this interaction which persist to all orders in perturbation theory. In addition to the expected renormalized M5{M}^{5}M5 contribution to the free energy, there are also terms linear in hhh (the magnetic field) and MMM which could be described as nonanalytic fluctuation-induced shifts in the order parameter and field. These might appear experimentally as apparent shifts in the background terms for hhh and MMM.
Finite-size and Fluctuation Effects on Phase Transition and Critical Phenomena using Mean-Field Approach Based on Renormalized ϕ4ϕ^{4}ϕ4 Model: I. Theory
An investigation of the spatial fluctuations and their manifestations in the vicinity of the quantum critical point within the framework of the renormalized ϕ4ϕ^{4}ϕ4 theory is proposed. Relevant features are reported through the Ginzburg-Landau-Wilson (GLW)-based calculations, combined with an efficient non perturbative technique. Both the dimension and size, but also microscopic details of the system, leading to critical behavior, and strongly deviating from the classical mean-field approach far from the thermodynamic limit, are taken into account. Further, the important role that harmonic and anharmonic fluctuations and finite-size effects can play in the determination of the characteristic properties of corresponding various systems, involving phase transitions and critical phenomena, is then discussed in detail with emphasis on the qualitative validity of the analysis
The crossover from ideal-Bose-gas behavior to nonclassical, XY-like criticality in a dilute, interacting Bose fluid is considered in detail with emphasis on the superfluid density, ${\ensuremath{\rho}}_{s}$(T), below the transition. A general discussion of crossover scaling in constrained systems shows that ${\ensuremath{\rho}}_{s}$ and other temperature-dependent thermodynamic functions at controlled overall density can be expressed in terms of universal scaling functions. The finite-temperature Green's function formalism is used to calculate the associated linear scaling fields and the crossover exponent exactly for all dimensionalities d>2, scaling predictions, including logarithmic singularities just at d=3, being checked via second-order computations. A mapping of the Bose problem onto the classical s4{s}^{4}s4 spin model is achieved which is asymptotically exact in the critical region. In this way, previously known renormalization-group results are used to calculate the scaling functions for ${\ensuremath{\rho}}_{s}$, for the off-diagonal susceptibility, etc., to first order in \ensuremath{\epsilon}=4-d. Good agreement is found with the experimental measurements by Reppy and co-workers of ${\ensuremath{\rho}}_{s}$(T) for helium absorbed at low coverages in Vycor glass (of pore size 50--80 A\r{}). An effective-mass ratio m∗{m}^{\mathrm{*}}m∗/m\ensuremath{\simeq}1.5, and an effective-scattering-length ratio a∗{a}^{\mathrm{*}}a∗/a\ensuremath{\simeq}1.3 are indicated for mobile helium in the Vycor (although the irregular character of Vycor has not, at this point, been fully allowed for in the theory). Other approaches in the literature to computing the critical behavior of quantum-mechanical systems are reviewed and related to the present analysis.
To set the stage for our subsequent thorough discussion of dynamic critical phenomena, we first review the theoretical description of second-order equilibrium phase transitions. (Readers already well acquainted with this material may readily move on to Chapter 2.) To this end, we compare the critical exponents following from the van-der-Waals equation of state for weakly interacting gases with the results from the Curie–Weiss mean-field approximation for the ferromagnetic Ising model. We then provide a unifying description in terms of Landau–Ginzburg theory, i.e., a long-wavelength expansion of the effective free energy with respect to the order parameter. The Gaussian model is analyzed, and a quantitative criterion is established that defines the circumstances when non-linear fluctuations need to be taken into account properly. Thereby we identify d c = 4 as the upper critical dimension for generic continuous phase transitions in thermal equilibrium. The most characteristic feature of a critical point turns out to be the divergence of the correlation length that renders microscopic details oblivious. As a consequence, not only the correlation functions, but remarkably the thermodynamics as well of a critical system are governed by an emergent unusual symmetry: scale invariance. A simple scaling ansatz is capable of linking different critical exponents; as an application, we introduce the basic elements of finite-size scaling. Finally, a brief sketch of Wilson's momentum shell renormalization group method is presented, intended as a pedagogical preview of the fundamental RG ideas. Exploiting the scale invariance properties at the critical point, the scaling forms of the free energy and the order parameter correlation function are derived.