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Physical Review A

Adiabatic elimination in stochastic systems. III. Application to renormalization-group transformations of the time-dependent Ginsburg-Landau model

Moira L. Steyn‐Ross, Crispin W. Gardiner

Published May 1, 1984• 11 citations
Publisher

Abstract

We present a stochastic theory of the renormalization-group transformations of the time-dependent Ginzburg-Landau model, describing a system which exhibits the characteristics of a second-order phase transition. We eliminate a certain range of the Fourier components of the magnetic spin variables via a projection-operator method. This effectively changes the scale of the system and transforms the coupling constants. The procedure is shown to be equivalent to the integration over short-wavelength modes as in the renormalization-group transformation performed by Wilson and Kogut. This equivalence shows that the projection-operator method is a valid procedure for scaling critical systems, and in particular it indicates that the treatment of fluctuations is systematic. We suggest that such a method should also provide a straightforward approach to the dynamic renormalization group.

DOI
10.1103/physreva.29.2834

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We present a stochastic theory of the renormalization-group transformations of the time-dependent Ginzburg-Landau model, describing a system which exhibits the characteristics of a second-order phase transition. We eliminate a certain range of the Fourier components of the magnetic spin variables via a projection-operator method. This effectively changes the scale of the system and transforms the coupling constants. The procedure is shown to be equivalent to the integration over short-wavelength modes as in the renormalization-group transformation performed by Wilson and Kogut. This equivalence shows that the projection-operator method is a valid procedure for scaling critical systems, and in particular it indicates that the treatment of fluctuations is systematic. We suggest that such a method should also provide a straightforward approach to the dynamic renormalization group.
The main idea of Wilson's renormalization group theory of critical phenomena is extended to the dynamical case. In the first place, a concept of dynamical fixed point is proposed. The dynamic scaling law is then derived from our new fixed-point hypothesis. Wilson's fixed-point hypothesis is also deduced from ours. Further, a method of renormalizing the master equation by eliminating the short wavelength modes is found. This is performed by means of the projection operator method. As an illustration, the dynamic critical exponents are calculated for a purely dissipative spin system. Related work based on the time-dependent Ginzburg-Landau equation is examined.
We discuss renormalization-group and mode-coupling theories of critical dynamics for several models. We show that to lowest order in $\ensuremath{\epsilon}={d}_{c}\ensuremath{-}d$, where for d>dcd>{d}_{c}d>dc​ conventional theory is valid, the renormalizationgroup differential equations are related by a simple transformation to those obtained from mode-coupling theory. Furthermore, the values of the dynamical fixed-point parameters essentially coincide with the critical amplitudes of the diverging transport coefficients, as determined by mode-coupling theory. In addition, we present a correct renormalization-group treatment for dynamical systems with more than one time scale, as for the binary liquid. We stress the fact that there are two distinct classes of critical dynamics, as illustrated by the results for the models studied here. We also discuss the instability of the conventional fixed point (time-dependent Ginzburg-Landau like) against mode-coupling perturbations for d<dcd<{d}_{c}d<dc​, with particular emphasis on a model for superfluid helium.
The renormalization-group method for studying critical phenomena is generalized to a class of dynamical systems---the time-dependent Ginzburg-Landau models. The effects of conservation laws on the critical dynamics are investigated through the study of models with different conservation properties for the energy and the space integral of the order parameter. Dynamic critical exponents near four dimensions ($d\ensuremath{\approx}4$) are obtained from recursion relations, analogous to those of Wilson and Fisher. The physical significance of the time-dependent Ginzburg-Landau models is explored and the applicability of the results to experiments on the NMR linewidth of FeF2{\mathrm{F}}_{2}F2​ is discussed.
For the n-vector model of the general type a renormalization-group (RG) scheme in momentum space is developed. Using this scheme we obtain a generalized exact equation of the renormalization group. The main idea of this approach is not to allow the zero-order critical Hamiltonian to move away from the critical surface under the influence of the RG transformation. The equation found differs from the known ones in that all redundant operators are excluded. On the basis of the developed approach we argue that critical exponents are functionals of some function introducing the cutoff into the phase-transition theory.
We apply Renormalization Group techniques to the Real Time formulation of thermal field theory. Due to the separation between the T = 0 and the T = 0 parts of the propagator in this formalism, one can derive exact evolution equations for the Green functions describing the effect of integrating out thermal fluctuations of increasing wavelengths, the initial conditions being the renormalized Green functions of the T = 0 theory. As a first application, we study the phase transition for the real scalar theory, computing the order of the transition, the critical temperature, and critical exponents, in different approximations to the evolution equations for the scalar potential. CERN-TH/96-23 The dynamics of a second order, or weakly first order, phase transition is governed by long wavelength fluctuations of the order parameter. These fluctuations, whose length scale is much larger than the inverse temperature of the system, are essentially classical, since the
The perturbative renormalization of the Ginzburg–Landau model is reconsidered based on the Feynman diagram technique. We derive renormalization group (RG) flow equations, exactly calculating all vertices appearing in the perturbative renormalization of the ' 4 model up to the 3 order of the -expansion. In this case, the ' 2 , ' 4 , ' 6 , and ' 8 vertices appear. All these are relevant in the sense that they do not vanish at the fixed point. We have tested the expected basic properties of the RG flow, such as the semigroup property. Under repeated RG transformation Rs, appropriately represented RG flow on the critical surface converges to certain s–independent fixed point. The Fourier–transformed two–point correlation function G(k) has been considered. Although the -expansion of X(k) = 1=G(k) is well defined on the critical surface, we have revealed a fundamental inconsistency of the perturbative method with the exact rescaling of X(k), represented as an expansion in powers of k at k ! 0. We discuss also certain paradox, related to the -expansion of X(k) above the critical point. Apart from the –expansion, we have tested also a modified approach, where the ' 4 coupling constant u is the expansion parameter at a fixed spatial dimensionality d. Our tests point to some internal inconsistency of such a method.