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Physical Review B

Optimizing Wilson’s momentum shell renormalization group

A. Tröster

Published Mar 31, 2010• 8 citations
Publisher

Abstract

Previous attempts to accurately compute critical exponents from Wilson's momentum shell renormalization prescription suffered from the difficulties posed by the presence of an infinite number of irrelevant couplings. Taking the example of the one-dimensional long-ranged Ising model, we calculate the momentum shell renormalization flow in the plane spanned by the coupling constants $({u}_{0},{r}_{0})$ for different values of the momentum shell thickness parameter $b$ by simulation using our recently developed Fourier Monte Carlo algorithm. We report strong anomalies in the $b$ dependence of the fixed-point couplings and the resulting exponents ${y}_{\ensuremath{\tau}}$ and $\ensuremath{\omega}$ in the vicinity of a shell parameter ${b}^{\ensuremath{\ast}}<1$ characterizing a thin but finite momentum shell. Evaluation of the exponents for this value of $b$ yields a dramatic improvement of their numerical accuracy, indicating a strong damping of the influence of irrelevant couplings for $b={b}^{\ensuremath{\ast}}$.

DOI
10.1103/physrevb.81.125135

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Previous attempts to accurately compute critical exponents from Wilson's momentum shell renormalization prescription suffered from the difficulties posed by the presence of an infinite number of irrelevant couplings. Taking the example of the one-dimensional long-ranged Ising model, we calculate the momentum shell renormalization flow in the plane spanned by the coupling constants (u0,r0)({u}_{0},{r}_{0})(u0​,r0​) for different values of the momentum shell thickness parameter bbb by simulation using our recently developed Fourier Monte Carlo algorithm. We report strong anomalies in the bbb dependence of the fixed-point couplings and the resulting exponents y\ensuremathτ{y}_{\ensuremath{\tau}}y\ensuremathτ​ and \ensuremathω\ensuremath{\omega}\ensuremathω in the vicinity of a shell parameter b\ensuremath∗<1{b}^{\ensuremath{\ast}}<1b\ensuremath∗<1 characterizing a thin but finite momentum shell. Evaluation of the exponents for this value of bbb yields a dramatic improvement of their numerical accuracy, indicating a strong damping of the influence of irrelevant couplings for b=b\ensuremath∗b={b}^{\ensuremath{\ast}}b=b\ensuremath∗.
Our recently developed Fourier Monte Carlo algorithm permits a nonperturbative calculation of momentum-shell renormalization-group flows by simulation which despite its apparent simplicity is illustrative both numerically as well as conceptually interesting. We study the example of a varphi(4) model with long-range lattice interaction. For this model we show that the topology of the renormalization flow is globally accessible in a particularly convenient way. The nontrivial fixed point of Wilson-Fisher type is observed its accompanying critical exponents are numerically determined from fitting its surrounding flow pattern to a linearized renormalization-group transformation. The results are compared to those obtained from perturbation theory, -expansion and earlier Monte Carlo simulations. Application of our method is also expected to be rewarding in other models with long-range interactions.
The computation of the critical exponent η characterizing the universal elastic behavior of crystalline membranes in the flat phase continues to represent challenges to theorists as well as computer simulators that manifest themselves in a considerable spread of numerical results for η published in the literature. We present additional insight into this problem that results from combining Wilson's momentum shell renormalization-group method with the power of modern computer simulations based on the Fourier Monte Carlo algorithm. After discussing the ideas and difficulties underlying this combined scheme, we present a calculation of the renormalization-group flow of the effective two-dimensional Young modulus for momentum shells of different thickness. Extrapolation to infinite shell thickness allows us to produce results in reasonable agreement with those obtained by functional renormalization group or by Fourier Monte Carlo simulations in combination with finite-size scaling. Moreover, our method allows us to obtain a decent estimate for the value of the Wegner exponent ω that determines the leading correction to scaling, which in turn allows us to refine our numerical estimate for η previously obtained from precise finite-size scaling data.
Wilson's momentum-shell transformation with the concomitant expansion around the upper critical dimension is defined. The basic notions of relevant and irrelevant couplings, renormalization flow, and fixed points are introduced. The origins of scaling and of universality are explained, and corrections to mean-field values of critical exponents are computed. The field theoretic renormalization group is briefly discussed and used to calculate the anomalous dimension. Idea We found that the direct perturbation theory in the Ginzburg–Landau–Wilson theory breaks down below the upper critical dimension because the perturbation parameter grows with the correlation length, and so becomes arbitrarily large as the critical point is approached. If the system were finite, on the other hand, the correlation length would be bound by its size, and perturbation theory could succeed. Singular thermodynamic behavior near the critical point comes from the thermodynamic limit, or, more precisely, from those modes that have arbitrary low energies in an infinitely large system. This is called the infrared singularity . This observation suggests the following strategy to avoid the problem of direct perturbation theory. First, note that mass m only provides the energy scale in Eq. (2.36). It is practical to rescale it out by absorbing it into the chemical potential, rede- fined as 2 m μ/ħ 2 → μ, and into the interaction coupling, as 2 m λ/ħ 2 → λ. Similarly, near the critical point, the temperature T ≈ T c may be replaced by the critical temperature, and then eliminated by rescaling the action as 2 m κ B T c S /ħ 2 → S.
Abstract Actual computations of fixed points and eigenvalues usually involve approximations, often crude ones, except for a very limited number of simple cases such as the one-dimensional Ising model of the previous chapter. In real- and momentum-space renormalization group theory, there are no general prescriptions to systematically improve the degree of the approximation with a modest amount of effort. There are established methods to systematically improve precision, but they usually need a large amount of numerical calculations. The scope of the present chapter is modest as we limit ourselves to basic examples, including the epsilon expansion about the Gaussian fixed-point of the Landau-Ginzburg-Wilson model. Finally, the last section illustrates the extension of the renormalization group framework to study quantum phase transitions.
In the finite-size scaling analysis of Monte Carlo data, instead of computing the observables at fixed Hamiltonian parameters, one may choose to keep a renormalization-group invariant quantity, also called phenomenological coupling, fixed at a given value. Within this scheme of finite-size scaling, we exploit the statistical covariance between the observables in a Monte Carlo simulation in order to reduce the statistical errors of the quantities involved in the computation of the critical exponents. This method is general and does not require additional computational time. This approach is demonstrated in the Ising model in two and three dimensions, where large gain factors in CPU time are obtained.
We calculate the critical exponent η of the D-dimensional Ising model from a simple truncation of the functional renormalization group flow equations for a scalar field theory with long-range interaction. Our approach relies on the smallness of the inverse range of the interaction and on the assumption that the Ginzburg momentum defining the width of the scaling regime in momentum space is larger than the scale where the renormalized interaction crosses over from long range to short range; the numerical value of η can then be estimated by stopping the renormalization group flow at this scale. In three dimensions our result η=0.03651 is in good agreement with recent conformal bootstrap and Monte Carlo calculations. We extend our calculations to fractional dimensions D and obtain the resulting critical exponent η(D) between two and four dimensions. For dimensions 2≤D≤3 our result for η is consistent with previous calculations.