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CRC Press eBooks

Phase Transitions – Validity of the Mean Field Theory – Scaling Laws

Nicolas Sator, Nicolas Pavloff, Lénaïc Couëdel

Published Jun 9, 2023• 0 citations
Publisher

Abstract

It has been seen in the previous chapters that Landau mean field approach relies on an incomplete partition function, the use of which is only justified if the fluctuations are Gaussian. This approach is manifestly invalid in the vicinity of a second order phase transition, since the second derivative of the Landau free energy with respect to the order parameter is equal to zero at the critical temperature https://www.w3.org/1998/Math/MathML" display="inline"> T C 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003272427/5ca4d37a-9786-43a3-8b38-8fa9eb3d1a19/content/math_26.tif "/> and the magnitude of the fluctuations near the critical point become prohibitive for the use of the theory. This is a real paradox, since Landau theory is designed to be valid exactly in the vicinity of second order phase transition points (see Section 9.2.2 ). The present chapter focuses first on the discussion of this problem and the precise evaluation of the domain of validity of mean field theories. In a second part, mean field predictions are compared with experimental data and the notion of scale invariance in the vicinity of the critical point is discussed.

DOI
10.1201/9781003272427-11

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The n -puzzle is a classical problem for modeling algorithms. For n -puzzle there are https://www.w3.org/1998/Math/MathML" display="inline"> n + 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_1.tif "/> different objects: n cells and one empty cell. Each object can be coded by https://www.w3.org/1998/Math/MathML" display="inline"> ρ = ⌈ log 2 ⁡ n + 1 ⌉ https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_2.tif "/> qubits and a configuration of https://www.w3.org/1998/Math/MathML" display="inline"> n + 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_3.tif "/> objects can be represented by a register of https://www.w3.org/1998/Math/MathML" display="inline"> z := ρ ⋅ ( n + 1 ) https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_4.tif "/> qubits https://www.w3.org/1998/Math/MathML" display="inline"> | x ⟩ https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_5.tif "/> . The function https://www.w3.org/1998/Math/MathML" display="inline"> g ( x ) https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_6.tif "/> is represented a unitary operator T . T acts on the https://www.w3.org/1998/Math/MathML" display="inline"> z + 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_7.tif "/> qubits with https://www.w3.org/1998/Math/MathML" display="inline"> x ∈ B z https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_8.tif "/> and https://www.w3.org/1998/Math/MathML" display="inline"> c ∈ B 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_9.tif "/> https://www.w3.org/1998/Math/MathML" display="block"> T ⋅ | x ⟩ | c ⟩ = | x ⟩ | f ( x ) ⊕ c ⟩ . https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_10.tif "/>

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Cambridge University Press eBooks•Jun 22, 1995•40 citations

Landau—Ginzburg and Fluctuations

Francesco Caravelli, Diego A. R. Dalvit

In the previous chapter, we developed the Landau theory of phase transitions, emphasizing how macroscopic order parameters and symmetry considerations determine the general form of the free energy near criticality. There, spatial variations of the order parameter were neglected, corresponding to the mean-field approximation . However, as we have seen, close to the critical temperature T c the correlation length ξ diverges, and fluctuations occur over arbitrarily large scales. In this regime, the assumption of spatial uniformity breaks down, and a local field description becomes necessary.

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It has been seen in the previous chapters that Landau mean field approach relies on an incomplete partition function, the use of which is only justified if the fluctuations are Gaussian. This approach is manifestly invalid in the vicinity of a second order phase transition, since the second derivative of the Landau free energy with respect to the order parameter is equal to zero at the critical temperature https://www.w3.org/1998/Math/MathML" display="inline"> T C 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003272427/5ca4d37a-9786-43a3-8b38-8fa9eb3d1a19/content/math_26.tif "/> and the magnitude of the fluctuations near the critical point become prohibitive for the use of the theory. This is a real paradox, since Landau theory is designed to be valid exactly in the vicinity of second order phase transition points (see Section 9.2.2 ). The present chapter focuses first on the discussion of this problem and the precise evaluation of the domain of validity of mean field theories. In a second part, mean field predictions are compared with experimental data and the notion of scale invariance in the vicinity of the critical point is discussed.
The nonequilibrium theory described in the previous chapters reduces to a particularly simple form for a system of fermions very close to zero temperature. To see this, let us define a “local occupation number” f ( p , ω ; R , T ) $ f(p, \omega ;R, T) $ https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315196596/a397f31e-8107-49bb-8694-b978d38168d6/content/inline-math12_1.tif"/> by writing 12.1 g < ( p , ω ; R , T ) = a ( p , ω ; R , T ) f ( p , ω ; R , T ) g<(p, ω;R, T)=a(p, ω;R, T)f(p, ω;R, T) g^{ < } (p,~\omega ;R,~T) = a(p,~\omega ;R,~T)f(p,~\omega ;R,~T) g<(p, ω;R, T)=a(p, ω;R, T)f(p, ω;R, T) https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781315196596/a397f31e-8107-49bb-8694-b978d38168d6/content/math12_1.tif"/>
Mean-field theory, presented in the preceding chapter, correctly describes the qualitative features of most phase transitions and, in some cases, the quantitative features. Since mean-field theory replaces the actual configurations of the local variables (e.g. spins) by their average value, it neglects the effects of fluctuations about this mean. These fluctuations may or may not be important. The more spins that interact with a particular test spin, the more the test spin sees an effective average or mean field. If the test spin interacts with two neighbors, the averaging is minimal and the fluctuations are large and important. The number of spins producing the effective field increases with the range of the interaction and with the dimension. Thus we will find that mean-field theory is a good approximation in high dimensions but fails to provide a quantitatively correct description of second-order critical points in low dimensions. This chapter will be devoted to the study of second-order phase transitions when mean-field theory is not a good approximation. We will begin by considering fluctuations of the order parameter about its spatially uniform mean-field value. We will find that for spatial dimensions below an upper critical dimension d c (typically d c = 4), fluctuations always become important and invalidate mean-field theory for temperatures sufficiently close to T c . This will motivate a generalization of Landau theory that incorporates spatial fluctuations about a mean-field free energy minimum.
Since the middle of the 90’s, it has been known how to trap and cool vapours of bosonic atoms (typically rubidium https://www.w3.org/1998/Math/MathML" display="inline"> 87 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003272427/5ca4d37a-9786-43a3-8b38-8fa9eb3d1a19/content/math_27.tif "/> Rb or sodium https://www.w3.org/1998/Math/MathML" display="inline"> 23 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003272427/5ca4d37a-9786-43a3-8b38-8fa9eb3d1a19/content/math_28.tif "/> Na) below the Bose–Einstein condensation temperature (see Section 7.4 ). These systems are very cold ( https://www.w3.org/1998/Math/MathML" display="inline"> T ∼ 100 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003272427/5ca4d37a-9786-43a3-8b38-8fa9eb3d1a19/content/math_29.tif "/> nK) and exhibit strong quantum features. They are also inhomogeneous (because the gas is confined by an external potential) and subject to non-negligible interaction effects: the condensed atoms accumulate at the bottom of the trapping potential well until reaching relatively high densities at which the physical properties of the system cannot always be described by the non-interacting model considered in Section 7.4 . The system’s low temperature is an asset for the theoretical treatment of the interactions because it is associated with a thermal wavelength that is larger than the typical distance between atoms: in this regime, it is legitimate to ignore the details of the interaction and to use a schematic potential, designed only to correctly describe the low energy scattering between two atoms. The simplicity of the resulting model enables to account, in the framework of a mean field approach, for non-trivial interaction effects for which it is possible to obtain precise experimental information.
The n -puzzle is a classical problem for modeling algorithms. For n -puzzle there are https://www.w3.org/1998/Math/MathML" display="inline"> n + 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_1.tif "/> different objects: n cells and one empty cell. Each object can be coded by https://www.w3.org/1998/Math/MathML" display="inline"> ρ = ⌈ log 2 ⁡ n + 1 ⌉ https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_2.tif "/> qubits and a configuration of https://www.w3.org/1998/Math/MathML" display="inline"> n + 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_3.tif "/> objects can be represented by a register of https://www.w3.org/1998/Math/MathML" display="inline"> z := ρ ⋅ ( n + 1 ) https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_4.tif "/> qubits https://www.w3.org/1998/Math/MathML" display="inline"> | x ⟩ https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_5.tif "/> . The function https://www.w3.org/1998/Math/MathML" display="inline"> g ( x ) https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_6.tif "/> is represented a unitary operator T . T acts on the https://www.w3.org/1998/Math/MathML" display="inline"> z + 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_7.tif "/> qubits with https://www.w3.org/1998/Math/MathML" display="inline"> x ∈ B z https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_8.tif "/> and https://www.w3.org/1998/Math/MathML" display="inline"> c ∈ B 1 https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_9.tif "/> https://www.w3.org/1998/Math/MathML" display="block"> T ⋅ | x ⟩ | c ⟩ = | x ⟩ | f ( x ) ⊕ c ⟩ . https://www.w3.org/1999/xlink" xlink:href=" https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781003374404/4745344c-4074-4386-85e6-ca8a3849de1f/content/math10_10.tif "/>
In the preceding two chapters, we have discussed various types of order that can occur in nature and how the ordering process can be quantified by the introduction of order parameters. We also developed a formalism for dealing with the thermodynamics of ordered states. In this chapter, we will use mean-field theory to study phase transitions and the properties of various ordered phases. Mean-field theory is an approximation for the thermodynamic properties of a system based on treating the order parameter as spatially constant. It is a useful description if spatial fluctuations are not important. It becomes an exact theory only when the range of interactions becomes infinite. It, nevertheless, makes quantitatively correct predictions about some aspects of phase transitions (e.g. critical exponents) in high spatial dimensions where each particle or spin has many nearest neighbors, and it makes qualitatively correct predictions in physical dimensions. Mean-field theory has the enormous advantage of being mathematically simple, and it is almost invariably the first approach taken to predict phase diagrams and properties of new experimental systems. Before proceeding, let us review some simple facts about phase transitions. At high temperatures, there is no order, and the order parameter 〈φ〉 is zero. At a critical temperature, T c , order sets in so that, for temperatures below T c , 〈φ〉 is nonzero. If 〈φ〉 rises continuously from zero, as shown in Fig. 4.0.1a, the transition is second order .
In the previous chapter, we developed the Landau theory of phase transitions, emphasizing how macroscopic order parameters and symmetry considerations determine the general form of the free energy near criticality. There, spatial variations of the order parameter were neglected, corresponding to the mean-field approximation . However, as we have seen, close to the critical temperature T c the correlation length ξ diverges, and fluctuations occur over arbitrarily large scales. In this regime, the assumption of spatial uniformity breaks down, and a local field description becomes necessary.