Mean field theory & universality

The number of degrees of freedom is conserved and the action is still manifestly gauge in-variant. Thus, the term “spontaneously broken gauge symmetry” should be read as “gauge symmetry/transformation becomes trivial”. Historically, the terms Anderson-Higgs mecha-nism and spontaneously broken gauge symmetry are used synonymously due to their formal equivalence. For an elucidating discussion on these matters see the excellent review about spontaneously violated gauge symmetries in superconductors [58].

Experimentally, there is strong evidence that our universe is actually in a condensed phase where a non-zero vacuum expectation value of a scalar field provides the electroweak symmetry break-ing mechanism in the standard model and thus gives rise to massive gauge bosons and generates masses of the elementary particles. Last year a new bosonic particle was discovered at the Large Hadron Collider in Geneva which seems to have the properties predicted by the standard model calculations. The mass of the particle is about126^GeV{^c2but if it is a scalar spin zero particle has yet to be ascertained.²³ If the new particle is the so-called Higgs particle, the collective excita-tion of the condensed scalar field, the particle/field content of the standard model of elementary particle physics would be complete. Studying the new particle more closely could yield more information about the properties of the (current) phase or state of our universe. In principle it could be possible that the Higgs field is of composite nature and the corresponding collective excitations are an emergent phenomenon.²⁴ Funnily enough, in condensed matter physics a Higgs-like mode has been detected (even prior to the discovery at CERN) in an quantum anti-ferromagnet displaying a dimerized disorder to magnetic long range order transition [61] and recently in an ultracold atoms system close to the critical point of the superfluid-insulator tran-sition [62]. A Higgs mode can only exist in a relativistic theory but in condensed matter the low-energy excitations are non relativistic (i.e. the mass gap is not equal to the dynamic mass of the excitation). However, in quantum phase transitions the ordered and disorder phase as well as the quantum critical point is described by relativistic Lagrangians. In addition to the gapless Goldstone modes, quantum critical systems possess gapped low-energy excitations with a mass gap proportional to the curvature of the minimized potential. In the superfluid-insulator exper-iment [62] the experexper-imental visibility is obscured due to the low-dimensionality of the system.

The Higgs mode can decay into multiple Goldstone modes and the respective amplitude of such a process in two dimensions has an infrared singularity.²⁵

outline the mean field methods and its breakdown which is connected to strong fluctuations destroying the mean field solution.

Mean Field Approximation

The central argument of the mean field theory relies on the fact that quantum fluctuations are irrelevant and the quantum operators can be effectively replace by classical complex numbers

∆

O

“

O

´ x

O

y ô

O

“ x

O

y `∆

O

.

The solution fixing the mean fieldx

O

ymust be found self-consistently. As the name suggest the complicated many-body Hamiltonian is reduced to a single quantum operator where the effect of the interaction/entanglement with all other operators is reduced to an averaged background field, the so-called mean field

O

¹

O

“

O

^1@

O

^D`@

O

^1D

O

´@

O

^{1D @}

O

^D`∆

O

¹∆

O

«

O

^1@

O

^D`@

O

^1D

O

.

This scheme can be carried over to field theoretic methods, where we introduce an additional fieldϕby means of the so-called Hubbard-Stratonovich transformation (being the inverse of the Gaussian identity (2.44))

exp ˆ

´1

2

O

i

V

ij

O

j

˙

«exp ˆ

´1 2

vΦ,

V

Φw˙

“

ż

Dϕexp ˆ

´1 2

vϕ ,

V

^´1ϕw´ipϕ , Φq

˙

(2.108) Note that the left-hand side of the Hubbard-Stratonovich transformation may involve a sum over discrete degrees of freedom, so in this case the averaging process can be accomplished by a single operation. This is not always possible. The integration over the original fields Φcomposed of complicated microscopic fields is now replaced by the integration over the auxiliary fieldϕ. We can view the Hubbard-Stratonovich transformation (2.108) as a decoupling of theΦinteraction by introducing the effect of the mean fieldϕon a single composite fieldΦ. The self-consistent mean field solution is determined by a stationary phase approximation (which is valid since the mean field is a averaged quantities over a large number of microscopic degrees of freedom pro-viding the existence of a large parameter),i.e.ϕ“ϕ^‹. Finally, the expansion about the stationary phase solution yields the relevant/important low-energy excitations above the ground state.

It is straight forward to describe critical phenomena using mean field methods. Since we are dealing with a low-energy effective theory encoding only the relevant information independent of the microscopic nature, we can immediately identify the mean field as the order parameter of a continuous phase transition. The ground or reference state mentioned above is identified with the stable phasee.g.ϕ^‹”0. Upon approaching the critical point, response functions will show singular behavior due to large fluctuations indicating the aforementioned instabilities²⁶. Thus, we are forced to adopt a new ground/reference state naturally arising from the new emergent minima in the field theory potential. This parallels exactly our discussion about spontaneously symmetry breaking in Section2.3.3. The new reference state will be characterized by a sponta-neously broken symmetric ground state with non-zero order parameter/mean fieldϕ^‹‰0. The

26Strictly speaking, as we will see at the end of this section, mean field theories fail to describe the critical point accurately, except above a certain spatial dimension called the upper critical dimensionDc.

Physical quantity Definition Behavior near critical point|T ´Tc|

T ąTc T ăTc

Susceptibility χ“ ´ δ²Frϕ;Js δJ²

ˇˇ ˇˇ

J“0

γ γ¹

Specific heat C“ ´T B²f

BT² α α¹

Order parameter xϕy “ δFrϕ;Js δJ

ˇˇ ˇˇ

J“0

pϕ^‹“0q β

Correlation length ξ ´ν ´ν¹

T ąTc T “Tc

Source field J pϕ^‹q^δ

Correlator xxϕpxqϕpyq yy |x´y|^{´D`1{2</sup>e<sup>´|x´y|{ξ</sup> |x´y|<sup>´D`2´η} Table 2.5. The free energy functional is defined as Frϕ;Js “ ´Tln

Z

rJs and the free en-ergy density f “ ^T{^LDSrϕ^‹;Js is an implicit function of the temperature. The order parameter is the thermal expectation value and may be sourced by an ex-ternal fieldJ. Although the connected correlation function of the order parameter Gϕpx,yq “ ´ xxϕpxqϕpyq yyis related to the response function/susceptibility by the fluctuation-dissipation theorem, the thermodynamic susceptibilityχencodes imag-inary time/thermal fluctuations only and not spatial fluctuations of the order pa-rameter field. The deeper understanding of the critical exponents will become clear when we will apply the renormalization group scheme in Section2.3.5.

low-energy excitations or Goldstone modes are the relevant low-energy excitations obtained by expanding about the non-zero stationary phase solution.

The microscopic theories are thus classified according to their universal low energy behavior en-coded in the critical exponents listed in Table2.5. All microscopic models that can be described by the same effective model/theory are said to be members of the same universality class,i.e. they possess the same universal properties (symmetries, dimensionalities, range of interactions,...) in-dependent of their microscopic origin. For each of the models in Table2.2and2.3we may find a universality class with unique critical exponents. For instance, the Landau-Ginzburg theory (sometimes calledϕ⁴theory or Mexican hat potential theory)

Srϕ;Js “

ż

d^Dx

„b

2p∇ϕq²`r 2ϕ<sup>2</sup>` u

4!ϕ⁴´ϕJ



(2.109) wherebdenotes the stiffness,²⁷ris the reduced temperature^pT´Tcq{T candudescribes effective interactions, is the effective field theory of the Ising universality class. Equation (2.109) can

27Strictly speaking, forb‰0we do note have a true mean field theory. The spatially uniform theory consisting solely of a free energy functional is usually called Landau theory, whereas the inclusion of spatial fluctuations is known as Landau-Ginzburg theory. In order to determineνandηand to understand the breakdown of the mean field theory it

Critical exponent D“1Ising D“2Ising D“3Ising (computed) Mean field theory

α 1 0plogq 0.110p1q 0

β 0 ¹{⁸ 0.3265p3q ¹{²

γ 1 ⁷{⁴ 1.2372p5q 1

δ 8 15 4.789p2q 3

ν 1 1 0.6301p4q ¹{²

η 1 ¹{⁴ 0.0364p5q 0

Table 2.6. The one and two dimensional Ising model can be solved analytically by duality techniques or the transfer matrix method. The three dimensional Ising model values are obtained form high temperature expansion and Monte Carlo methods taken from [67] and are in close agreement with experimental results. As can be seen, the critical exponents depend on the dimensionality of the system. For increasing dimensionality the system approaches the mean field behavior due to the decreasing importance of fluctuations (see also InfoboxGinzburg Criterionon page52).

be “derived” from the Ising Hamiltonian by applying a Hubbard-Stratonovich transformation (2.108) and takeing a successive continuum limit. The Ising universality class describes phase transitions in ferromagnetic materials and the liquid gas phase transition, whereas the superflu-id/normal fluid phase transition is effectively described by the XY model,i.e. it belongs to the XY model universality class. A very comprehensive and detailed overview concerning the proper-ties of theOpnquniversality classes and classification of physical phase transitions can be found in [67]. The critical exponents of the mean field theory are derived by inserting the saddle point solution

δS δϕ ˇˇ ˇˇ

ϕ“ϕ^‹

“ ´b∇²ϕ^‹`rϕ<sup>‹</sup>`u

6pϕ^‹q³´J “^! 0 (2.110)

J“0

ùùñ

$&

%

ϕ^‹“0, rą0, pT ąTcq

|ϕ^‹|“ c

´6|r|

u , ră0, pT ăTcq , (2.111)

displaying degenerate minima, into the Landau-Ginzburg action (2.109)

Srϕ^‹s “ L^D T

´r

2|ϕ^‹|²` u

4!|ϕ^‹|⁴´ϕ^‹J

¯

“ L^D T

$&

%

0, rą0, pT ąT_cq

´3 2

r²

u, ră0, pT ăT_cq (2.112) We see that for T ă T_c the energetic preferred configuration f “ ´^3r2{^2u is the asymmetric solution with finite order parameter. Applying all definitions of Table2.5 to (2.110), (2.111) and (2.112) yields the mean field critical exponents shown in Table2.6.

Unfortunately, the mean field theory has some serious drawbacks, most importantly it fails to reproduce computational and experimental results in two and three spatial dimensions (as can

is instructive to include these fluctuations. In Section2.3.5we will see that the correlation length scales likeξ„? b so the microscopic interaction range is related tobmotivating the colloquial term “stiffness”.

be seen in Table 2.6) and even worse it predicts the existence of (finite temperature) phase transitions in one and two dimensional systems. The reasons for this failure lies in the relevance of fluctuations in low-dimensional systems which destroys the naïve mean field solution. In order to estimate the range of applicability of the mean field approximation we require the fluctuations to be sufficiently smaller than the mean field “strength”. This is the so-called Ginzburg criterion:

Ginzburg Criterion

The mean field approximation yields a self-consistent solution if the correlated spatial fluc-tuations about the mean fieldx∆ϕpxq∆ϕpyq yare sufficiently small

G_ϕpx,yq ! xϕy²

In Fourier representation the Landau Ginzburg correlator close to the critical point reads G_ϕpx´yq „

ż d^Dk p2πq^D

e^ik¨px´yq r`bq²

rÑ0

ÝÝÝÑG_ϕpξq „

ż d^Dpkξq p2πq^D

ξ²

1`bpkξq<sup>2</sup>e<sup>ipkξq</sup> „ξ<sup>´D`2</sup> where we neglected the interaction term due to its irrelevance and we assume that all dimen-sionful quantities scale with the correlated fluctuations over the length scale|x´y|“ ξ.²⁸ Written in terms of critical exponents

Gϕ„r^pD´2qν!r^2β „ xϕy² ÝÑ pD´2qν ą2β

the Ginzburg criterion states that the mean field approximation is only consistent in D ą 2^β{^γ`2. In particular for the Ising universality class we findDą4.

In general,i.e. for all universality classes above the respective upper critical dimensionDc, fluc-tuations can be neglected and the mean-field approximation yields a valid solution. The same holds true for quantum fluctuations and quantum critical points at zero temperature. Only for dimensions larger than the upper critical dimension the quantum fluctuations can be neglected and the semi-classical approximation is valid. Therefore, only forDąD_cthe semi-classical the-ory describes quantum critical points correctly. There is also a lower critical dimension usually denoted byD_ℓ. Only systems with larger dimension then D_ℓ exhibit finite temperature phase transitions. Heuristically, this dimension can already be seen from the expression of the correla-tor in Table2.5

G_ϕpx,yq „|x´y|^´D`2´η for T «T_c (2.113) If we neglect the so-called “anomalous” scaling dimension²⁹η we see that forD “2 the long-range correlations in the broken/condensed phase are virtually non-existent, as well as the asso-ciated long-range order.

28For a full justification of the scaling behavior and irrelevance of interactions see the following Section2.3.5on renor-malization.

29Strictly speaking a misnomer, since it only hints at another important microscopic length scale besides the correlation length which is connected to ultra-violet divergences. In principle all other critical exponents may be corrected by additional “anomalous” coefficient which is usually not represented by a special symbol. From the renormalization perspective (developed in Section2.3.5) the “anomalous” scaling dimensions arise from the influence of irrelevant operators/fields which might be still important close to the critical point,e.g. correlations on all length scales imply contributions of the microscopic lattice spacing or the short wavelength cut-off.

Coleman-Mermin-Wagner Theorem

A continuous symmetry cannot be broken spontaneously in physical systems with sufficiently short-ranged interactions. This implies that there are no finite temperature phase transitions due to the existence of long-range fluctuations favoring the disordered high-temperature phase, thus destroying long-range order. The Landau Ginzburg correlator for fluctuations about the vacuum state reads

xδϕpxqδϕpyq y “

ż d^Dq p2πq^D

e^iq¨px´yq q² „

ż₈

0

dq q^D´1 q²

The small momenta long-wavelength (infrared) divergence, due to quantum fluctuations of the massless Nambu-Goldstone modes, persists only in systems with spatial dimensions D ď2. A proper and well-explained derivation of this theorem from a statistical viewpoint can be found in [68].

For systems with discrete symmetry breaking we can use Peierl’s argument to obtain the lower critical dimensionD_ℓ“1. Comparing energy cost and entropy gain, we see that the free energy of a disordered discrete one-dimensional system is always favored at finite temperature since the entropy is logarithmically diverging with the system size. But for a two-dimensional system the energy cost for introducing disorder scales with the length of the domain wallL, separat-ing different degenerate ground state configurations, as does the entropy. Thus, the free energy difference of the ordered and disordered phase approaches zero at a finite critical temperature.

Quantum fluctuations cannot alter this result since we do not have massless modes connecting the different ground states, as in the continuous symmetry case. For completeness let us men-tion again that the lower critical dimension of gauge/symmetries is formally8due to Elitzur’s theorem.

Im Dokument Gauge/Gravity duality (Seite 68-73)