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Journal of Physics C Solid State Physics

Some invariance properties of the renormalization group

Franz Wegner

Published Jun 21, 1974• 157 citations
Publisher

Abstract

A generalization of Wilson's renormalization group equation is discussed. A large class of equivalent fixed points exists for each fixed point. The eigen perturbations fall into two classes: (i) scaling operators whose scaling exponents are independent of the fixed point within the class of equivalent fixed points, and (ii) redundant operators whose exponents depend on the choice of the renormalization group equation. The exponents of the redundant operators are spurious since the free energy depends only on the scaling fields of the scaling operators but not on the scaling fields of the redundant operators.

DOI
10.1088/0022-3719/7/12/004

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A generalization of Wilson's renormalization group equation is discussed. A large class of equivalent fixed points exists for each fixed point. The eigen perturbations fall into two classes: (i) scaling operators whose scaling exponents are independent of the fixed point within the class of equivalent fixed points, and (ii) redundant operators whose exponents depend on the choice of the renormalization group equation. The exponents of the redundant operators are spurious since the free energy depends only on the scaling fields of the scaling operators but not on the scaling fields of the redundant operators.
An invariance theorem for exponents of scaling variables is proved for a class of renormalization groups introduced by Wegner. (Renormalization groups generated by a flow vector.) A convenient form for the renormalization group linearized both around the fixed point and around the renormalization trajectory is derived. The latter is used to derive a coordinate system for perturbations around a critical Hamiltonian from that around a fixed point. It is shown that the classification scaling redundant applies also to the vectors of this critical-point coordinate system. The effect of an infinitesimal perturbation of the flow vector on the fixed point and on the renormalization group linearized around the fixed point is investigated. If the unperturbed linearized group has no marginal scaling operator, two cases arise. (i) The perturbed group has no fixed point near that of the unperturbed group. This case may obtain if there is a marginal redundant observable. (ii) The perturbed group has a fixed point which differs from that of the unperturbed group by a redundant perturbation. In the second case the scaling eigenvalues of the perturbed linearized group will be unchanged. Eigenvalues of redundant observables may change. The effect of a perturbation in a flow vector on a renormalized trajectory is considered and shown to suggest the concept of manifolds of equivalent Hamiltonians. Some difficulties involved in this concept are discussed.
Abstract Chapter 5 first recalls the importance of the concept of scale decoupling in physics. It then emphasizes that quantum field theory and the theory of critical phenomena have provided two examples where this concepts fails. To deal with such a situation, a new tool has been invented: the renormalization group. In the framework of effective quantum field theory, a perturbative renormalization group has been formulated. Its implementation has led to the discovery of fixed points as zeros of beta functions, and calculations of critical exponents of a class of macroscopic phase transitions in the form of Wilson–Fisher epsilon or fixed dimension expansions. These expansions being divergent, they could summed by methods based on the Borel transformation and the determination of the large order behaviour of perturbation theory.
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.
A class of renormalization groups characterized as the general Gaussian renormalization group is defined. It includes the sharp cutoff renormalization group of Wilson, the linear renormalization group of Bell and Wilson, and the incomplete‐integration renormalization group of Wilson. The renormalization group approach to critical phenomena and the expansion around four dimensions is reviewed. The fixed point of the general Gaussian renormalization group is determined to order ε2, where ε=4−d (d is the dimensionality of the system), and the critical exponents are determined to the same order. The main conclusion is that the exponents do not depend on the cutoff functions of the renormalization group.
The large distance behaviour, near the critical temperature, of continuous phase transitions with short range interactions can be reproduced by an effective local field theory. Within this framework, the universal properties of the large distance can be determined by finding fixed points of general, functional renormalization group (RG) equations (Wilson, Wegner). For dimension D > 4, the large scale behavior is governed by the Gaussian fixed point (related to mean field theory), while for D < 4 the Gaussian fixed point is unstable. Near D = 4, the RG equations can be solved in the form of an ǫ = 4 − D expansion, a new stable fixed point is found and this leads to the famous Wilson-Fisher expansion for critical exponents and other universal quantities. Moreover, one realizes that, in this framework, the effective field theory can be reduced to a renormalizable field theory and, at leading order in the critical domain, the much simpler quantum field theory renormalization group can be used. A number of physicists, including the Saclay group (Brezin, Le Guillou, Zinn-Justin) have developed sophisticated field theory techniques, to prove general scaling laws and to calculate exponents to higher orders [1].