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Physical Review Letters

Next-to-Leading Terms in the Correlation Function inside the Scaling Regime

E. Brézin, D J Amit, Jean Zinn‐Justin

Published Jan 28, 1974• 54 citations
Publisher

Abstract

An application of Symanzik's method for treating exceptional momenta (the Callan-Symanzik equations combined with operator-product expansions) yields the leading corrections to the large-momentum behavior of the correlation function near the critical point. The result is used to argue that the conclusion drawn from numerical calculations, namely, that strong scaling is violated, is unwarranted.

DOI
10.1103/physrevlett.32.151

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An application of Symanzik's method for treating exceptional momenta (the Callan-Symanzik equations combined with operator-product expansions) yields the leading corrections to the large-momentum behavior of the correlation function near the critical point. The result is used to argue that the conclusion drawn from numerical calculations, namely, that strong scaling is violated, is unwarranted.
Using renormalization group arguments the author expands n-point correlation functions (for non-exceptional wavevectors) in expectation values of translational invariant short-range operators O i . He uses the fact that the Fourier components of our operators become negligible for wavevectors q large in comparison to the momentum cut-off. The correlation functions show the same non-analyticities at the critical point as the expectation values (O i ). The expansion coefficients are regular in the thermodynamic variables for q not=0. They can be expressed in terms of (a) functions which become singular at q=0 and yield the scaling behaviour, and (b) functions which are regular at q=0. The expansion coefficients of the two-point correlation function are sums of both types of functions.
A detailed calculation to order ${\ensuremath{\epsilon}}^{2}$ of the two-point correlation function in the zero-field critical region ($T\ensuremath{\ge}{T}_{c}$) is presented for an nnn-vector system in $d=4\ensuremath{-}\ensuremath{\epsilon}$ dimensions. Scaling behavior is verified over the full parameter range and the scaling function is obtained as a universal explicit convergent integral (independent of any cutoffs). Study of the scaling function as $T\ensuremath{\rightarrow}{T}_{c}$ in the finite ($q\ensuremath{\ne}0$) momentum limit, confirms the presence of terms varying as ${t}^{1\ensuremath{-}\ensuremath{\alpha}}$ and as ttt [with $t=\frac{(T\ensuremath{-}{T}_{c})}{{T}_{c}}$]; explicit evaluation of their amplitudes reliably determines the form of the scattering intensity at fixed qqq and locates the corresponding maximum accurately.
In a companion paper [Blaizot, Phys. Rev. E 74, 051116 (2006)], we have presented an approximation scheme to solve the nonperturbative renormalization group equations that allows the calculation of the n-point functions for arbitrary values of the external momenta. The method was applied in its leading order to the calculation of the self-energy of the O(N) model in the critical regime. The purpose of the present paper is to extend this study to the next-to-leading order of the approximation scheme. This involves the calculation of the four-point function at leading order, where interesting features arise, related to the occurrence of exceptional configurations of momenta in the flow equations. These require a special treatment, inviting us to improve the straightforward iteration scheme that we originally proposed. The final result for the self-energy at next-to-leading order exhibits a remarkable improvement as compared to the leading order calculation. This is demonstrated by the calculation of the shift DeltaTc, caused by weak interactions, in the temperature of Bose-Einstein condensation. This quantity depends on the self-energy at all momentum scales and can be used as a benchmark of the approximation. The improved next-to-leading order calculation of the self-energy presented in this paper leads to excellent agreement with lattice data and is within 4% of the exact large N result.
The zero-field two-point correlation function of an nnn-vector system in ddd dimensions is calculated to order 1n\frac{1}{n}n1​ for $T\ensuremath{\ge}{T}_{c}$ and 2<d<42<d<42<d<4. The critical-point scaling function is obtained as a closed cutoff-independent integral. As $t=\frac{(T\ensuremath{-}{T}_{c})}{{T}_{c}}\ensuremath{\rightarrow}0$ at fixed wave vector qqq, the variation of the Fourier transform of the correlation function is ${\stackrel{^}{E}}_{1}(q){t}^{1\ensuremath{-}\ensuremath{\alpha}}+{\stackrel{^}{E}}_{2}(q)t+{\stackrel{^}{E}}_{3}(q){t}^{2}+\ensuremath{\cdots}$, where $\ensuremath{\alpha}$ is the specific-heat exponent and ${\stackrel{^}{E}}_{2}$, ${\stackrel{^}{E}}_{3}$ are of order 1n\frac{1}{n}n1​. At d=3d=3d=3 the term t2{t}^{2}t2 is modified by logarithmic corrections. As $q\ensuremath{\rightarrow}0$ at fixed nonzero ttt, deviations of the scaling function from the Ornstein-Zernike form ${(1+{\ensuremath{\xi}}^{2}{q}^{2})}^{\ensuremath{-}1}$ are of order $\frac{{10}^{\ensuremath{-}3}{\ensuremath{\xi}}^{4}{q}^{4}}{n}$. Results are compared with high-temperature series-expansion estimates and with the $\ensuremath{\epsilon}$-expansion results of previous work.
In this paper, we analyze the renormalization group (RG) flow of field theories with quenched disorder, in which the couplings vary randomly in space.We analyze both classical (Euclidean) disorder and quantum disorder, emphasizing general properties rather than specific cases.The RG flow of the disorderaveraged theories takes place in the space of their coupling constants and also in the space of distributions for the disordered couplings, and the two mix together.We write down a generalization of the Callan-Symanzik equation for the flow of disorder-averaged correlation functions.We find that local operators can mix with the response of the theory to local changes in the disorder distribution and that the generalized Callan-Symanzik equation mixes the disorder averages of several different correlation functions.For classical disorder, we show that this can lead to new types of anomalous dimensions and to logarithmic behavior at fixed points.For quantum disorder, we find that the RG flow always generates a rescaling of time relative to space, which at a fixed point generically leads to Lifshitz scaling.The dynamical scaling exponent z behaves as an anomalous dimension (as in other nonrelativistic RG flows), and we compute it at leading order in perturbation theory in the disorder for a general theory.Our results agree with a previous perturbative computation by Boyanovsky and Cardy, and with a holographic disorder computation of Hartnoll and Santos.We also find in quantum disorder that local operators mix with nonlocal (in time) operators under the RG, and that there are critical exponents associated with the disorder distribution that have not previously been discussed.In large-N theories, the disorder averages may be computed exactly, and we verify that they are consistent with the generalized Callan-Symanzik equations.