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Physical review. B, Condensed matter

Light scattering spectrum of He4 for T>~Tλ in three dimensions

Pierre C. Hohenberg, Sarben Sarkar

Published Oct 1, 1981• 10 citations
Publisher

Abstract

The field-theoretic renormalization-group formalism is used to calculate the light scattering spectrum of $^{4}\mathrm{He}$ for $T>~{T}_{\ensuremath{\lambda}}$ self-consistently to second order in the loop expansion. It is shown that owing to the finite wave vector of the experiments the system never comes close to its fixed point (even for $T={T}_{\ensuremath{\lambda}}$), and transients must be included consistently to explain the data. Previous explanations of the absence of temperature dependence of the linewidth, in terms of the effect of the weak-scaling fixed point, are shown to be incorrect.

DOI
10.1103/physrevb.24.3800

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The field-theoretic renormalization-group formalism is used to calculate the light scattering spectrum of 4He^{4}\mathrm{He}4He for T> T\ensuremathλT>~{T}_{\ensuremath{\lambda}}T> T\ensuremathλ​ self-consistently to second order in the loop expansion. It is shown that owing to the finite wave vector of the experiments the system never comes close to its fixed point (even for T=T\ensuremathλT={T}_{\ensuremath{\lambda}}T=T\ensuremathλ​), and transients must be included consistently to explain the data. Previous explanations of the absence of temperature dependence of the linewidth, in terms of the effect of the weak-scaling fixed point, are shown to be incorrect.
A nonlinear renormalization-group (RG) analysis of the thermal conductivity data near the $\ensuremath{\lambda}$ line of 4He^{4}\mathrm{He}4He is performed on the basis of the corrected dynamic RG flow equations of model FFF in two-loop order. Our analysis eliminates recent uncertainties in the regime $(T\ensuremath{-}{T}_{\ensuremath{\lambda}})∕{T}_{\ensuremath{\lambda}}\ensuremath{\lesssim}{10}^{\ensuremath{-}6}$, which is most sensitive to the effect of the weak-scaling fixed point. This effect is found to be stronger than described previously. We also improve the Borel summation of the amplitude of the specific heat.
We study perturbative Wilsonian renormalization group (RG) for the scalar ϕ 4 theory at finite temperature to one loop order in the Schwinger-Keldysh closed-time-path (CTP) formalism. By explicitly integrating out the UV modes, we show how effective interactions coupling the two branches of the CTP contour arise. This provides a microscopic derivation of the type of effective CTP actions used in the literature. While the full effective theory has no critical points, we instead proceed to study critical properties of the RG flow in a reduced space of complex couplings that characterize certain time-ordered correlators and show the existence of two novel fixed points in d = 4 spacetime dimensions. We relate one to the Wilson-Fisher fixed point known to exist only in 4 − ε dimensions, and the other to the standard Gaussian fixed point.
The dynamic renormalization-group flow equations for model F of Halperin, Hohenberg, and Siggia [Phys. Rev. B 13, 1299 (1976)] are calculated by means of renormalized field theory within the minimal-renormalization scheme up to two-loop order. These equations are combined with the Borel-resummation results for the static renormalization-group functions computed by Schloms and Dohm. The corresponding static fixed point destabilizes the dynamic-scaling fixed point in two-loop order. The nonuniversal initial values of the static and dynamic flow equations are identified for the \ensuremath{\lambda} transition of 4He^{4}\mathrm{He}4He at various pressures. Predictions are made for the bulk thermal conductivity very close to ${\mathit{T}}_{\ensuremath{\lambda}}$ where the departures from dynamic scaling should be observable. Effective static and dynamic parameters are computed that can be applied to other critical phenomena above and below the \ensuremath{\lambda} line of 4He^{4}\mathrm{He}4He.
We develop a theoretical framework based on the nonperturvative renormalization group (RG) in the one-particle irreducible (Wetterich) formulation to tackle the interplay of coupled fermionic and order-parameter fluctuations at metallic quantum critical points (QCPs) with ordering wavevector Q⃗=0⃗\vec{Q}=\vec{0}Q​=0. We consistently treat the dynamical emergence of the Landau damping of the bosonic mode and non-Fermi liquid scaling of fermions upon lowering the cutoff scale. The loop integrals of the present theory involve only contributions from fluctuations above the cutoff scale, which protects the system from developing singular bosonic interactions. We emphasize the importance of the nontrivial relative scaling of the bosonic and fermionic cutoffs ΛΛΛ and ΛfΛ_fΛf​, which we fix by analyzing the RG flow of the scale-dependent ordering wave-vector Q⃗Λ\vec{Q}_ΛQ​Λ​. Upon neglecting Fermi self-energy in the loop integrals of the functional RG flow, we identify a non-Fermi liquid RG fixed point and recover the features obtained earlier within RPA-type approaches. In a subsequent step, we self-consistently include the scaling of the self-energy and the Yukawa coupling. We find a generic instability of the non-Fermi liquid RG fixed point. This implies, at least at this truncation level, absence of the QCP with Q⃗=0⃗\vec{Q}=\vec{0}Q​=0 and development of a first-order phase transition or a phase characterized by Q⃗≠0⃗\vec{Q}\neq\vec{0}Q​=0.
The Renormalization group method (RG) is applied to the investigation of the E model of critical dynamics, which describes the transition from the normal to the superfluid phase in He4. The technique “Sector decomposition” with R’ operation is used for the calculation of the Feynman diagrams. The RG functions, critical exponents and critical dynamical exponent z, which determines the growth of the relaxation time near the critical point, have been calculated in the two-loop approximation in the framework of ε-expansion. The relevance of a fixed point for helium, where the dynamic scaling is weakly violated, is briefly discussed.