Functional renormalization group equation for strongly correlated fermions

collective degrees of freedom

Hubbard model Electrons on a cubic lattice Electrons on a cubic lattice here : on planes ( d = 2 ) here : on planes ( d = 2 ) Repulsive local interaction if two electrons are on the same site Repulsive local interaction if two electrons are on the same site Hopping interaction between two neighboring sites Hopping interaction between two neighboring sites

In solid state physics : “ model for everything “ Antiferromagnetism Antiferromagnetism High T c superconductivity High T c superconductivity Metal-insulator transition Metal-insulator transition Ferromagnetism Ferromagnetism

Hubbard model Functional integral formulation U > 0 : repulsive local interaction next neighbor interaction External parameters T : temperature μ : chemical potential (doping )

Fermi surface Fermion quadratic term ω F = (2n+1)πT Fermi surface : zeros of P for T=0

Antiferromagnetism in d=2 Hubbard model temperature in units of t antiferro- magnetic order parameter T c /t = U/t = 3

Collective degrees of freedom are crucial ! for T < T c nonvanishing order parameter nonvanishing order parameter gap for fermions gap for fermions low energy excitations: low energy excitations: antiferromagnetic spin waves antiferromagnetic spin waves

QCD : Short and long distance degrees of freedom are different ! Short distances : quarks and gluons Short distances : quarks and gluons Long distances : baryons and mesons Long distances : baryons and mesons How to make the transition? How to make the transition? confinement/chiral symmetry breaking confinement/chiral symmetry breaking

Nambu Jona-Lasinio model …and more general quark meson models

Chiral condensate (N f =2) Chiral condensate (N f =2)

Functional Renormalization Group from small to large scales from small to large scales

How to come from quarks and gluons to baryons and mesons ? How to come from electrons to spin waves ? Find effective description where relevant degrees of freedom depend on momentum scale or resolution in space. Find effective description where relevant degrees of freedom depend on momentum scale or resolution in space. Microscope with variable resolution: Microscope with variable resolution: High resolution, small piece of volume: High resolution, small piece of volume: quarks and gluons quarks and gluons Low resolution, large volume : hadrons Low resolution, large volume : hadrons

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Effective potential includes all fluctuations

Scalar field theory linear sigma-model for chiral symmetry breaking in QCD or: scalar model for antiferromagnetic spin waves (linear O(3) – model ) fermions will be added later fermions will be added later

Scalar field theory Scalar field theory

Flow equation for average potential

Simple one loop structure –nevertheless (almost) exact

Infrared cutoff

Partial differential equation for function U(k,φ) depending on two ( or more ) variables Z k Z k = c k -η

Regularisation For suitable R k : Momentum integral is ultraviolet and infrared finite Momentum integral is ultraviolet and infrared finite Numerical integration possible Numerical integration possible Flow equation defines a regularization scheme ( ERGE –regularization ) Flow equation defines a regularization scheme ( ERGE –regularization )

Integration by momentum shells Momentum integral is dominated by q 2 ~ k 2. Flow only sensitive to physics at scale k

Wave function renormalization and anomalous dimension for Z k (φ,q 2 ) : flow equation is exact !

Effective average action and exact renormalization group equation

Generating functional Generating functional

Loop expansion : perturbation theory with infrared cutoff in propagator Effective average action

Quantum effective action

Exact renormalization group equation

Exact flow equation for effective potential Evaluate exact flow equation for homogeneous field φ. Evaluate exact flow equation for homogeneous field φ. R.h.s. involves exact propagator in homogeneous background field φ. R.h.s. involves exact propagator in homogeneous background field φ.

Flow of effective potential Ising model CO 2 T * = K p * =73.8.bar ρ * = g cm-2 Experiment : S.Seide … Critical exponents

Antiferromagnetic order in the Hubbard model A functional renormalization group study T.Baier, E.Bick, …

Temperature dependence of antiferromagnetic order parameter temperature in units of t antiferro- magnetic order parameter T c /t = U = 3

Mermin-Wagner theorem ? No spontaneous symmetry breaking of continuous symmetry in d=2 ! of continuous symmetry in d=2 !

Fermion bilinears Introduce sources for bilinears Functional variation with respect to sources J yields expectation values and correlation functions

Partial Bosonisation collective bosonic variables for fermion bilinears collective bosonic variables for fermion bilinears insert identity in functional integral insert identity in functional integral ( Hubbard-Stratonovich transformation ) ( Hubbard-Stratonovich transformation ) replace four fermion interaction by equivalent bosonic interaction ( e.g. mass and Yukawa terms) replace four fermion interaction by equivalent bosonic interaction ( e.g. mass and Yukawa terms) problem : decomposition of fermion interaction into bilinears not unique ( Grassmann variables) problem : decomposition of fermion interaction into bilinears not unique ( Grassmann variables)

Partially bosonised functional integral equivalent to fermionic functional integral if Bosonic integration is Gaussian or: solve bosonic field equation as functional of fermion fields and reinsert into action

fermion – boson action fermion kinetic term boson quadratic term (“classical propagator” ) Yukawa coupling

source term is now linear in the bosonic fields

Mean Field Theory (MFT) Evaluate Gaussian fermionic integral in background of bosonic field, e.g.

Effective potential in mean field theory

Mean field phase diagram μ μ TcTc TcTc

Mean field inverse propagator for spin waves T/t = 0.5T/t = 0.15 P m (q) Baier,Bick,…

Mean field ambiguity TcTc μ mean field phase diagram U m = U ρ = U/2 U m = U/3,U ρ = 0 Artefact of approximation … cured by inclusion of bosonic fluctuations J.Jaeckel,…

Flow equation for the Hubbard model T.Baier, E.Bick, …

Truncation Concentrate on antiferromagnetism Potential U depends only on α = a 2

scale evolution of effective potential for antiferromagnetic order parameter boson contribution fermion contribution effective masses depend on α ! gap for fermions ~α

running couplings

unrenormalized mass term Running mass term four-fermion interaction ~ m -2 diverges -ln(k/t)

dimensionless quantities renormalized antiferromagnetic order parameter κ

evolution of potential minimum -ln(k/t) U/t = 3, T/t = 0.15 κ λ

Critical temperature For T size of probe > 1 cm -ln(k/t) κ T c =0.115 T/t=0.05 T/t=0.1

Pseudocritical temperature T pc Limiting temperature at which bosonic mass term vanishes ( κ becomes nonvanishing ) It corresponds to a diverging four-fermion coupling This is the “critical temperature” computed in MFT !

Pseudocritical temperature T pc μ TcTc MFT(HF) Flow eq.

critical behavior for interval T c < T < T pc evolution as for classical Heisenberg model cf. Chakravarty,Halperin,Nelson

critical correlation length c,β : slowly varying functions exponential growth of correlation length compatible with observation ! at T c : correlation length reaches sample size !

critical behavior for order parameter and correlation function

Bosonic fluctuations fermion loops boson loops mean field theory

Rebosonisation adapt bosonisation to every scale k such that adapt bosonisation to every scale k such that is translated to bosonic interaction is translated to bosonic interaction H.Gies, … k-dependent field redefinition absorbs four-fermion coupling

Modification of evolution of couplings … Choose α k such that no four fermion coupling is generated Evolution with k-dependent field variables Rebosonisation

…cures mean field ambiguity TcTc U ρ /t MFT Flow eq. HF/SD

Nambu Jona-Lasinio model

Critical temperature, N f = 2 J.Berges,D.Jungnickel,… Lattice simulation

Chiral condensate Chiral condensate

temperature dependent masses temperature dependent masses pion mass sigma mass

Critical equation of state

Scaling form of equation of state Berges, Tetradis,…

Universal critical equation of state is valid near critical temperature if the only light degrees of freedom are pions + sigma with O(4) – symmetry. Not necessarily valid in QCD, even for two flavors !

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