Functional renormalization with interaction flows: A single-boson exchange perspective and application to electron-phonon systems
Pith reviewed 2026-05-17 20:10 UTC · model grok-4.3
The pith
Single-boson exchange lets fRG regulate interactions as well as propagators.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Starting from Schwinger--Dyson and Bethe--Salpeter equations, we develop here an fRG formulation where both bare propagators and bare interactions can be dressed with regulators. The approach thus obtained is an extension of the multiloop fRG recently introduced for many-fermion systems. Using the single-boson exchange decomposition, we show that the underlying flow equations are simply interpreted as adding a regulator to the bosonic propagator and that such an extension scarcely changes the original structure of the flow equations.
What carries the argument
Single-boson exchange decomposition, which recasts the regulated interaction flow as a simple regulator added to the bosonic propagator.
If this is right
- Temperature flows become feasible for models that include retarded interactions.
- Loop convergence can be compared directly to conventional cutoff schemes in the Anderson impurity model.
- A cutoff applied simultaneously to the propagator and the bare interaction is shown to be valid for an Anderson impurity coupled to a phonon.
- Classes of renormalization schemes previously inaccessible with standard fRG methods are now reachable.
Where Pith is reading between the lines
- The method may simplify renormalization flows in any system whose interactions are carried by a bosonic mode.
- It could improve treatment of finite-temperature effects in models with frequency-dependent interactions.
- Analogous regulator placements might be explored for other interaction decompositions in many-body calculations.
Load-bearing premise
The single-boson exchange decomposition remains accurate when regulators are placed on interactions.
What would settle it
Numerical flows for the Anderson impurity model in which the new interaction-regulated temperature scheme deviates strongly from established benchmarks or from conventional propagator-only cutoff results.
Figures
read the original abstract
The functional renormalization group (fRG) is acknowledged as a powerful tool in quantum many-body physics and beyond. On the technical side, conventional implementations of the fRG rely on regulators for bare propagators only. Starting from Schwinger--Dyson and Bethe--Salpeter equations, we develop here an fRG formulation where both bare propagators and bare interactions can be dressed with regulators. The approach thus obtained is an extension of the multiloop fRG recently introduced for many-fermion systems. Using the single-boson exchange decomposition, we show that the underlying flow equations are simply interpreted as adding a regulator to the bosonic propagator and that such an extension scarcely changes the original structure of the flow equations. Overall, we provide a framework for implementing approaches that cannot be realized with conventional fRG methods, such as temperature flows for models with retarded interactions. For concrete applications, we analyze the loop convergence of our scheme against conventional cutoff schemes for the Anderson impurity model. Finally, we devise a new temperature-flow scheme that implements a cutoff in both the propagator and the bare interaction, and demonstrate its validity on a model of an Anderson impurity coupled to a phonon.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript develops an extension of multiloop functional renormalization group (fRG) methods that incorporates regulators on both bare propagators and bare interactions. Starting from the Schwinger-Dyson and Bethe-Salpeter equations and employing the single-boson exchange (SBE) decomposition of the four-point vertex, the authors argue that the resulting flow equations reduce to the addition of a regulator solely to the bosonic propagator, with negligible alteration to the original flow structure. Concrete applications include tests of loop convergence against conventional cutoff schemes in the Anderson impurity model and the implementation of a new temperature-flow scheme for an Anderson impurity coupled to a phonon.
Significance. If the central claim holds, the work enables fRG flows (such as temperature flows) in systems with retarded interactions that are inaccessible to conventional propagator-only regulators. The SBE perspective is presented as preserving the simplicity of the multiloop structure, and the numerical demonstrations on impurity models provide a practical test bed. This could broaden the applicability of fRG to electron-phonon and related condensed-matter problems.
major comments (1)
- [Derivation using SBE decomposition and electron-phonon application] The central claim that the SBE decomposition allows the regulated interaction flow to be interpreted simply as adding a regulator to the bosonic propagator (without substantial structural change or extra vertex corrections) is load-bearing for the entire extension. This assumption is not isolated and verified in the electron-phonon application, where frequency-dependent bare interactions could mix with the bosonic self-energy flow and generate additional regulator-dependent corrections that would require re-deriving the multiloop equations. The Anderson-impurity loop-convergence tests do not address this mixing.
minor comments (1)
- [Abstract] The abstract states that the extension 'scarcely changes the original structure of the flow equations'; a short explicit statement of which equations remain unchanged would improve clarity for readers unfamiliar with the multiloop fRG baseline.
Simulated Author's Rebuttal
We thank the referee for their careful reading of our manuscript and for providing constructive feedback. We address the major comment below and are willing to revise the manuscript accordingly to strengthen the presentation.
read point-by-point responses
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Referee: [Derivation using SBE decomposition and electron-phonon application] The central claim that the SBE decomposition allows the regulated interaction flow to be interpreted simply as adding a regulator to the bosonic propagator (without substantial structural change or extra vertex corrections) is load-bearing for the entire extension. This assumption is not isolated and verified in the electron-phonon application, where frequency-dependent bare interactions could mix with the bosonic self-energy flow and generate additional regulator-dependent corrections that would require re-deriving the multiloop equations. The Anderson-impurity loop-convergence tests do not address this mixing.
Authors: We thank the referee for this insightful comment. Our derivation, based on the Schwinger-Dyson and Bethe-Salpeter equations combined with the single-boson exchange (SBE) decomposition, demonstrates that regulating the bare interaction is equivalent to regulating the bosonic propagator. This equivalence is general and does not depend on the specific form of the interaction, including frequency-dependent cases. In the electron-phonon application, the bare interaction is the phonon-mediated retarded interaction, which is incorporated as the bosonic propagator in the SBE framework. The flow equations for the vertex and self-energies follow the standard multiloop structure, with the regulator on the bosonic propagator accounting for the interaction flow. Any potential mixing with the bosonic self-energy is already included in the flow and does not generate additional vertex corrections that would necessitate re-deriving the equations. The Anderson impurity tests establish the reliability of the multiloop convergence, while the electron-phonon example validates the temperature flow for retarded interactions. To explicitly address the referee's concern and isolate this verification, we will add a dedicated subsection or appendix in the revised manuscript that derives the flow equations specifically for the electron-phonon model, confirming the absence of extra corrections. revision: yes
Circularity Check
Derivation self-contained from SDE/BSE without reduction to inputs or self-citations
full rationale
The paper begins from the standard Schwinger-Dyson and Bethe-Salpeter equations and extends the existing multiloop fRG framework via the single-boson exchange decomposition. The claim that interaction regulators reduce to a bosonic-propagator regulator follows directly from applying the decomposition to the regulated equations; this is a structural consequence of the chosen vertex parametrization rather than a redefinition or fit of the target result inside the paper. No load-bearing self-citation chains, ansatzes smuggled via prior work, or predictions that are statistically forced by construction appear in the derivation steps described. The Anderson-impurity and electron-phonon applications serve as numerical checks rather than inputs that define the central result. The derivation therefore remains independent of its own outputs.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Single-boson exchange decomposition accurately represents the interaction vertex for the models under study.
Reference graph
Works this paper leans on
- [1]
-
[2]
J. M. Pawlowski, Aspects of the functional renormali- sation group, Annals of Physics322, 2831 (2007)
work page 2007
-
[3]
P. Kopietz, L. Bartosch, and F. Sch¨ utz,Introduction to the Functional Renormalization Group, Lecture Notes in Physics (Springer, Berlin, 2010)
work page 2010
-
[4]
B. Delamotte, An introduction to the nonperturbative renormalization group, inRenormalization Group and Effective Field Theory Approaches to Many-Body Sys- tems, edited by A. Schwenk and J. Polonyi (Springer Berlin Heidelberg, Berlin, Heidelberg, 2012) pp. 49–132
work page 2012
-
[5]
W. Metzner, M. Salmhofer, C. Honerkamp, V. Meden, and K. Sch¨ onhammer, Functional renormalization group approach to correlated fermion systems, Rev. Mod. Phys.84, 299 (2012)
work page 2012
-
[6]
N. Dupuis, L. Canet, A. Eichhorn, W. Metzner, J. M. Pawlowski, M. Tissier, and N. Wschebor, The nonper- turbative functional renormalization group and its ap- plications, Physics Reports910, 1 (2021), the nonper- turbative functional renormalization group and its ap- plications
work page 2021
-
[7]
Wetterich, Exact evolution equation for the effective potential, Physics Letters B301, 90 (1993)
C. Wetterich, Exact evolution equation for the effective potential, Physics Letters B301, 90 (1993)
work page 1993
-
[8]
Ellwanger, Flow equations forNpoint functions and bound states, Z
U. Ellwanger, Flow equations forNpoint functions and bound states, Z. Phys. C - Particles and Fields62, 503 (1994)
work page 1994
-
[9]
T. R. Morris, The Exact Renormalization Group and Approximate Solutions, International Jour- nal of Modern Physics A09, 2411 (1994), https://doi.org/10.1142/S0217751X94000972
-
[10]
F. B. Kugler and J. von Delft, Multiloop Functional Renormalization Group That Sums Up All Parquet Di- agrams, Phys. Rev. Lett.120, 057403 (2018)
work page 2018
-
[11]
F. B. Kugler and J. von Delft, Multiloop functional renormalization group for general models, Phys. Rev. B97, 035162 (2018)
work page 2018
-
[12]
F. B. Kugler and J. von Delft, Derivation of exact flow equations from the self-consistent parquet relations, New Journal of Physics20, 123029 (2018)
work page 2018
-
[13]
J.-P. Blaizot, J. M. Pawlowski, and U. Reinosa, Exact renormalization group and Φ-derivable approximations, Physics Letters B696, 523 (2011). 31
work page 2011
-
[14]
J.-P. Blaizot, J. M. Pawlowski, and U. Reinosa, Func- tional renormalization group and 2PI effective action formalism, Annals of Physics431, 168549 (2021)
work page 2021
-
[15]
J. Thoenniss, M. K. Ritter, F. B. Kugler, J. von Delft, and M. Punk, Multiloop pseudofermion func- tional renormalization for quantum spin systems: Ap- plication to the spin-1 2 kagome Heisenberg model (2020), arXiv:2011.01268 [cond-mat.str-el]
- [16]
-
[17]
M. K. Ritter, D. Kiese, T. M¨ uller, F. B. Kugler, R. Thomale, S. Trebst, and J. von Delft, Benchmark cal- culations of multiloop pseudofermion fRG, Eur. Phys. J. B95, 102 (2022)
work page 2022
-
[18]
P. Chalupa-Gantner, F. B. Kugler, C. Hille, J. von Delft, S. Andergassen, and A. Toschi, Fulfillment of sum rules and Ward identities in the multiloop functional renormalization group solution of the Anderson impu- rity model, Phys. Rev. Res.4, 023050 (2022)
work page 2022
-
[19]
A. Ge, N. Ritz, E. Walter, S. Aguirre, J. von Delft, and F. B. Kugler, Real-frequency quantum field theory applied to the single-impurity Anderson model, Phys. Rev. B109, 115128 (2024)
work page 2024
-
[20]
N. Ritz, A. Ge, E. Walter, S. Aguirre, J. von Delft, and F. B. Kugler, KeldyshQFT: A C++ codebase for real- frequency multiloop functional renormalization group and parquet computations of the single-impurity An- derson model, The Journal of Chemical Physics161, 054118 (2024)
work page 2024
-
[21]
A. Tagliavini, C. Hille, F. B. Kugler, S. Andergassen, A. Toschi, and C. Honerkamp, Multiloop functional renormalization group for the two-dimensional Hubbard model: Loop convergence of the response functions, Sci- Post Phys.6, 009 (2019)
work page 2019
- [22]
- [23]
-
[24]
S. Heinzelmann, A. Toschi, and S. Andergassen, En- tangled magnetic, charge, and superconducting pair- ing correlations in the two-dimensional Hubbard model: a functional renormalization-group analysis (2023), arXiv:2308.06497 [cond-mat.str-el]
-
[25]
C. Hille,The role of the self-energy in the functional renormalization group description of interacting Fermi systems, Ph.D. thesis, Universit¨ at T¨ ubingen (2020)
work page 2020
-
[26]
N. E. Bickers and D. J. Scalapino, Critical behavior of electronic parquet solutions, Phys. Rev. B46, 8050 (1992)
work page 1992
-
[27]
A. Al-Eryani, Diagrammatic bosonisation, aspects of criticality, and the Hohenberg-Mermin-Wagner Theo- rem in parquet approaches (2025), arXiv:2509.15094
-
[28]
W. Metzner and D. Vollhardt, Correlated Lattice Fermions ind=∞Dimensions, Phys. Rev. Lett.62, 324 (1989)
work page 1989
-
[29]
A. Georges, G. Kotliar, W. Krauth, and M. J. Rozen- berg, Dynamical mean-field theory of strongly corre- lated fermion systems and the limit of infinite dimen- sions, Rev. Mod. Phys.68, 13 (1996)
work page 1996
-
[30]
C. Taranto, S. Andergassen, J. Bauer, K. Held, A. Katanin, W. Metzner, G. Rohringer, and A. Toschi, From Infinite to Two Dimensions through the Func- tional Renormalization Group, Phys. Rev. Lett.112, 196402 (2014)
work page 2014
-
[31]
D. Vilardi, C. Taranto, and W. Metzner, Antiferromag- netic andd-wave pairing correlations in the strongly interacting two-dimensional Hubbard model from the functional renormalization group, Phys. Rev. B99, 104501 (2019)
work page 2019
-
[32]
P. M. Bonetti, A. Toschi, C. Hille, S. Andergassen, and D. Vilardi, Single-boson exchange representation of the functional renormalization group for strongly inter- acting many-electron systems, Phys. Rev. Research4, 013034 (2022)
work page 2022
- [33]
-
[34]
J. L. Smith and Q. Si, Spatial correlations in dynamical mean-field theory, Phys. Rev. B61, 5184 (2000)
work page 2000
-
[35]
R. Chitra and G. Kotliar, Effect of Long Range Coulomb Interactions on the Mott Transition, Phys. Rev. Lett. 84, 3678 (2000)
work page 2000
-
[36]
R. Chitra and G. Kotliar, Effective-action approach to strongly correlated fermion systems, Phys. Rev. B63, 115110 (2001)
work page 2001
- [37]
-
[38]
A. A. Katanin, Extended dynamical mean field the- ory combined with the two-particle irreducible func- tional renormalization-group approach as a tool to study strongly correlated systems, Phys. Rev. B99, 115112 (2019)
work page 2019
-
[39]
C. Honerkamp and M. Salmhofer, Temperature-flow renormalization group and the competition between su- perconductivity and ferromagnetism, Phys. Rev. B64, 184516 (2001)
work page 2001
-
[40]
A for generic models with quartic interations
We illustrate this point in more detail in App. A for generic models with quartic interations
-
[41]
N. Dupuis, Nonperturbative renormalization-group approach to fermion systems in the two-particle- irreducible effective action formalism, Phys. Rev. B89, 035113 (2014)
work page 2014
-
[42]
J. Polonyi and K. Sailer, Effective action and density- functional theory, Phys. Rev. B66, 155113 (2002)
work page 2002
-
[43]
A. Schwenk and J. Polonyi, Towards Density Functional Calculations from Nuclear Forces (2004), arXiv:nucl- th/0403011 [nucl-th]
- [44]
-
[45]
Renormalization, Conservation Laws and Transport in Correlated Electron Systems
T. Enss,Renormalization, Conservation Laws and Transport in Correlated Electron Systems, Ph.D. thesis, Universit¨ at Stuttgart (2005), arXiv:cond-mat/0504703 [cond-mat.str-el]
work page internal anchor Pith review Pith/arXiv arXiv 2005
-
[46]
J. F. Rentrop, S. G. Jakobs, and V. Meden, Two- particle irreducible functional renormalization group schemes—a comparative study, Journal of Physics A: Mathematical and Theoretical48, 145002 (2015). 32
work page 2015
-
[47]
J. F. Rentrop, V. Meden, and S. G. Jakobs, Renor- malization group flow of the Luttinger-Ward functional: Conserving approximations and application to the An- derson impurity model, Phys. Rev. B93, 195160 (2016)
work page 2016
-
[48]
K. Fraboulet and J.-P. Ebran, Addressing energy den- sity functionals in the language of path-integrals II: comparative study of functional renormalization group techniques applied to the (0+0)-D O(N)-symmetricφ 4- theory, Eur. Phys. J. A60, 45 (2024)
work page 2024
-
[49]
S. Kemler and J. Braun, Towards a renormalization group approach to density functional theory—general formalism and case studies, Journal of Physics G: Nu- clear and Particle Physics40, 085105 (2013)
work page 2013
- [50]
- [51]
- [52]
- [53]
-
[54]
T. Yokota and T. Naito, Functional-renormalization- group aided density functional analysis for the corre- lation energy of the two-dimensional homogeneous elec- tron gas, Phys. Rev. B99, 115106 (2019)
work page 2019
-
[55]
T. Yokota and T. Naito, Ab initio construction of the energy density functional for electron systems with the functional-renormalization-group-aided den- sity functional theory, Phys. Rev. Res.3, L012015 (2021)
work page 2021
- [56]
- [57]
- [58]
-
[59]
K. Fraboulet, S. Heinzelmann, P. M. Bonetti, A. Al- Eryani, D. Vilardi, A. Toschi, and S. Andergassen, Single-boson exchange functional renormalization group application to the two-dimensional Hubbard model at weak coupling, Eur. Phys. J. B95, 202 (2022)
work page 2022
-
[60]
K. Fraboulet, A. Al-Eryani, S. Heinzelmann, A. Kauch, and S. Andergassen, Multiloop functional renormaliza- tion group from single bosons (2025)
work page 2025
-
[61]
M. Gievers, E. Walter, A. Ge, J. von Delft, and F. B. Kugler, Multiloop flow equations for single-boson ex- change fRG, Eur. Phys. J. B95, 108 (2022)
work page 2022
-
[62]
P. W. Anderson, Localized Magnetic States in Metals, Phys. Rev.124, 41 (1961)
work page 1961
-
[63]
R. E. Lagos, The Holstein–Anderson impurity model, Journal of Magnetism and Magnetic Materials226-230, 103 (2001), proceedings of the International Conference on Magnetism (ICM 2000)
work page 2001
-
[64]
A. C. Hewson and D. Meyer, Numerical renormalization group study of the Anderson-Holstein impurity model, Journal of Physics: Condensed Matter14, 427 (2001)
work page 2001
-
[65]
N. E. Bickers, Self-Consistent Many-Body Theory for Condensed Matter Systems, inS´ en´ echal D., Tremblay A.-M., Bourbonnais C. (eds), Theoretical Methods for Strongly Correlated Electrons, CRM Series in Mathe- matical Physics(Springer, New York, 2004)
work page 2004
-
[66]
G. Rohringer, H. Hafermann, A. Toschi, A. A. Katanin, A. E. Antipov, M. I. Katsnelson, A. I. Lichtenstein, A. N. Rubtsov, and K. Held, Diagrammatic routes to nonlocal correlations beyond dynamical mean field the- ory, Rev. Mod. Phys.90, 025003 (2018)
work page 2018
-
[67]
N. E. Bickers and S. R. White, Conserving approxima- tions for strongly fluctuating electron systems. II. Nu- merical results and parquet extension, Phys. Rev. B43, 8044 (1991)
work page 1991
-
[68]
Gievers,Functional approaches to Fermi polarons in cold atomic gases and solid-state systems, Ph.D
M. Gievers,Functional approaches to Fermi polarons in cold atomic gases and solid-state systems, Ph.D. thesis, Ludwig-Maximilians-Universit¨ at M¨ unchen (2025)
work page 2025
-
[69]
We refer to App. C.2 of Ref. [68] for more details on the construction of the identity vertices1 r and1 r
-
[70]
A. Al-Eryani, S. Heinzelmann, K. Fraboulet, F. Krien, and S. Andergassen, Screening and effective RPA-like charge susceptibility in the extended Hubbard model (2024), arXiv:2412.07323 [cond-mat.str-el]
-
[71]
A. Al-Eryani, S. Andergassen, and M. M. Scherer, Inter- twined fluctuations and isotope effects in the Hubbard- Holstein model on the square lattice from functional renormalization, Phys. Rev. Res.7, 043052 (2025)
work page 2025
-
[72]
The subtraction ofF r from∇ (F) r is needed because M(B) r only exhibits two-particle-reducible diagrams
-
[73]
[61], can then be directly recovered from our developments of Sec
The derivation of the original SBE equations (12), as outlined in Ref. [61], can then be directly recovered from our developments of Sec. II C 2 by settingF r = 0 (and thereforeB r =U)
-
[74]
Y. Zhang and J. Callaway, Extended Hubbard model in two dimensions, Phys. Rev. B39, 9397 (1989)
work page 1989
-
[75]
H. Terletska, T. Chen, and E. Gull, Charge ordering and correlation effects in the extended Hubbard model, Phys. Rev. B95, 115149 (2017)
work page 2017
-
[76]
H. Terletska, T. Chen, J. Paki, and E. Gull, Charge or- dering and nonlocal correlations in the doped extended Hubbard model, Phys. Rev. B97, 115117 (2018)
work page 2018
-
[77]
J. Paki, H. Terletska, S. Iskakov, and E. Gull, Charge order and antiferromagnetism in the extended Hubbard model, Phys. Rev. B99, 245146 (2019)
work page 2019
-
[78]
Holstein, Studies of polaron motion: Part I
T. Holstein, Studies of polaron motion: Part I. The molecular-crystal model, Annals of Physics8, 325 (1959)
work page 1959
-
[79]
J. K. Freericks and M. Jarrell, Competition between Electron-Phonon Attraction and Weak Coulomb Repul- sion, Phys. Rev. Lett.75, 2570 (1995)
work page 1995
- [80]
discussion (0)
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