Workshop at Yukawa Institute, Kyoto, Japan
Renormalization Group Approach from Ultra Cold
Atoms to the Hot QGP, August 22- September 9,2011

Non-equilibrium time evolution ofbosons from the functional
renormalization group
Peter Kopietz, Universität Frankfurt
collaboration: T. Kloss (Grenoble), A. Kreisel, J. Hick (Frankfurt)

A. Serga, B. Hillebrands (Kaiserslautern, experiment)

talk based on:

outline:Eur. Phys. J. B 71, 59 (2009)●Phys. Rev. B 81,104308 (2010)●Phys. Rev. B 83, 205118 (2011)●

1. Functional integral formulation of Keldysh technique
2. Non­equilibrium FRG
3. Parametric resonance of magnons in Yttrium­Iron­Garnet1
4. FRG for toy model

warning!
D.Mermin, Physics Today,November 1992, page 9:“It is absolutely impossible to give too elementary a physics
talk. Every talk I have even attended in four decades oflecture going has been too hard.There is therefore no point
in advising you to make your talk clear and comprehensible.
You should merely strive to place as far as possible fromthe beginning the grim moment when more than 90% of
your audience is able to make sense of less than 10% ofanything you say.”2

1. functional integral formulation of
the Keldysh technique(A. Kamenev, Les Houches, 2004)●

model: magnons coupled to phonons:

magnon●phononnon-equilibrium Green functions:3

Keldysh contour and classical/quantum
components●

Keldysh contour:change of basis: fromcontour labels to classicaland quantum labels:●

functional integral
representation of nonequilibrium Greenfunctions:●4

Keldysh action in continuum notation●

QQ-blocks ofGaussian propagators
are infinitesimal
regularization:5

non-equilibrium time-evolution:
quantum kinetic equations
Keldysh component of non-equilibrium Dyson equation
gives kinetic equation for distribution function:●Green function matrix:●

matrices in momentum and time

self-energy matrix:●

subtracting left/right Dyson eqns. gives kinetic eq.:●

6different forms of the kinetic equation
1.) time-domain:2.) with subtractions to identify collision integrals:7

....different forms of the kinetic equation
3.) for distribution function:4.) for distribution function, Wigner transformed:

8

2. Non-equilibrium renormalizationgroup
several numerical implementations in condensed matter:

●1.)

2.)

9

different versions of numerical non-equilibrum RG3.)

4.)(flow equationsfor continuousunitarytransfomations)10

functional renormalization group●exact equation for change of generating
functional of irreducible vertices as IR
cutoff is reduced (Wetterich 1993)●

exact RG flow equations for all vertices●

flow of self-energy:

11non-equilibrium time-evolution from the FRG
the basic idea●

introduce cutoff parameter

which somehow simplifies time evolution●

write down suitably truncated FRG flow equations for the self-energies●

structure of resulting equations:coupled system of partial
integro-differential equations
in two variables●

make standard approximations to simplify system(e.g. reduction to Fokker-Planck eq)
or solve by brute force numerically.
12FRG cutoff schemes to calculate
non-equilibrium time-evolution
requirments: cutoff procedure should respect:●1) causality, and 2) in equilibrium: fluctuation-dissipation theorem

possibilies:●

1) long-time cutoff (Gasenzer, Pawlowski, 2008)2) hybridization cutoff (Jakobs, Pletyukhov, Schoeller, 2010)3) out-scattering rate cutoff (Kloss, P.K., 2011)13

non-equilibrium FRG vertex expansion(Gezzi et al, 2007; Gasenzer+Pawlowski, 2008; Kloss+PK 2010)

simple generalization of the equilibrium vertex expansion:●

14

3. Parametric resonance of magnons
in yttrium-iron-garnet (YIG)●●

what is YIG?
ferromagneticinsulator
at the first sight:
too complicated!
A. Kreisel, F. Sauli,
L. Bartosch, PK, 2009●

effective quantum spin model for relevant magnon band:exchange interaction:

saturation magnetization:

lattice spacing:effective spin:

15

experiments on YIG: probing the non-equilibrium
dynamics of magnons●

motivation:

collaboration withexperimental groupof B. Hillebrands(Kaiserslautern)non-equilibrium
dynamics of interacting
magnons in YIG●

experiment:

microwave-pumping of
magnons in YIG
measurement ofmagnon distriubution
via Brillouin light scattering

16

from spin operators to bosons:
Holstein-Primakoff transformation●

problem: spin-algebra is very complicated:●

solution: for ordered magnets: bosonization of spins (Holstein,Primakoff 1940)

spin algebra indeed satisfied if●

proof that different dimension of Hilbert spaces does not matter by Dyson 1956:●17some history: magnon dynamics in YIG
H. Suhl, 1957, E. Schlömann et al, 1960s,
V. E. Zakharov, V. S. L’vov, S. S. Starobinets, 1970s●

minimal model:●“S-theory”: time-dependent self-consistent HartreeFock approximation for magnon distributions functions
weak points: ●no microscopic description of dissipation and damping●possibility of BEC not included!

●goals:consistent quantum kinetic theory for magnons in YIG beyond Hartree-Fock●include time-evolution of Bose-condendsate●develop functional renormalization group for non-equilibrium
18●

magnon dispersion of finite YIG films●

dispersion of lowest magnon mode has minimum at finite k
due to interplay between: 1. exchange interaction
2. dipole-dipole interaction
3. finite width of films

19toy model for dynamics of lowest magnon mode
T. Kloss, A. Kreisel, PK, PRB 2010●

keep only lowest magnon mode
anharmonic oscillator with
off-diagonal pumping:●rotating reference frame:●

instability of non-interacting system for large pumping:for●oscillator has negative mass

non-interacting hamiltonian not positive definite●

parametric resonance20

parametric resonance●what is parametric resonance?
classical harmonic oscillator withharmonic frequency modulation:●●

resonance condition:
oscillator absorbs energy at a rate
proportional to the energy it already has!

●history:●

discovered: Melde experiment, 1859
excite oscillations of string by periodicallyvarying its tension at twice its resonance frequency
theoretically explained: Rayleigh 1883●

21

time-dependent Hartree-Fock approximation(“S-theory”)●

order parameter●

Gross-Pitaevskii equation:connected correlation functions:kinetic equations:●order parameter:

Hamiltonian dynamics
in effective potential(Hartree-Fock)22

4. Time-evolution of the toy modelfrom the FRG(T. Kloss, P.K., Phys. Rev. B, 2011)●

toy model can be solved numerically exactly bysolving time-dependent Schrödinger equation●

generalize FRG to include also off-diagonal Green functions:●

Keldysh component at equal times gives distribution functions:23

time-dependent Hartree-Fock approximation●

coupled kinetic equations for diagonal/off-diagonal distribution:(“S-theory, V. E. Zakharov, V. S. L’vov, S. Starobionets, 1970s)●matrix notation:●

how good is time-dependent Hartree-Fock?

24

time-dependent Hartree-Fock: comparison withexact non-equilibrium dynamics
small interaction:

larger interaction:

diagonal
correlator:

off-diagonalcorrelator:

25

cutoff scheme for non-equilibrium FRG●

toy model has no intrinsic dissipation (oscillatory dynamics,no relaxation to equilibrium)●out-scattering cutoff scheme gives best results

still infinitesimal regularization●for FRG need single-scale propagators:

26

first order truncation of the FRG hierarchy

27

comparison of first order FRG withexact non-equilibrium dynamics
small interaction:

larger interaction:

diagonal
correlator:

off-diagonalcorrelator:

28

Summary+Outlook●

FRG approach to non-equilibrium dynamics of bosons●

cutoff procedures: out-scattering versus hybridization●

exactly solvable toy model for magnon dynamics in YIG:
first order truncation of FRG quite good!

Outlook:●relaxation towards equilibrium: rate equations with scaledependent transition rates●quantum kinetics of BEC from the FRG●

parametric resonance in YIG: beyond S-theory.

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