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Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Spontaneous Symmetry Breaking and

Goldstone Modes

Soren Petrat

May 26, 2009

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Outline

1 Spontaneous Symmetry BreakingSymmetries in PhysicsThe phenomenon of SSB

2 Implications of SSBGoldstone ModesGoldstone TheoremMermin-Wagner Theorem

3 Conclusion

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

What is a symmetry in Physics?

Invariance of a physical law under transformations

Symmetries can arise from physics . . .

e.g. Galilei-invariance in Newtonian Mechanics

. . . or from mathematics

e.g. gauge-invariance in Electrodynamics:AA + fx

Symmetries form a group

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

Different kinds of symmetries

we differentiate between:

global symm.: acts simultaneously on all variables

local symm.: acts independently on each variable

furthermore:

continuous symm., e.g. rotations (SO(n))discrete symm., e.g. spin group (Z2)

Lie group: differentiable manifold that is also a grouprespecting the continuum properties of the manifold

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

The concept of SSB

Observation: our world is (mostly) not symmetric!

General concept in modern physicsoriginal law is symmetric

but the solutions are not!

i.e. the symmetry is broken (by some mechanism realizedin our world)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

Examples

Unification of fundamental forces in particle physics

early universe: only one forceuniverse cooled down separation to 4 fundamentalforces

origin: superconductivity (Anderson 1958)

today: applications in condensed matter physics(superconductivity, superfluidity, BEC) and QFT (particlephysics, Standard Model)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

Definition of SSB

example: Ising model

H0 = 1

2

ij

iJijj (1)

i = 1, i Zd , d: dimension, no external field hereH0 is Z2-invariant (Z2: global discrete symm.)if Jij = J(|Ri Rj |) lattice symm.: Zd (Bravais)we know: phase transitions for d 2order parameter: magnetization m

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

Definition of SSB

H0 is Z2-inv., but solution for m not: it changes sign!mean-field: m | |, mf = 12 for T TcHelmholtz free energy a(m,T )

former symm. is broken!Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

Definition of SSB

The general concept:

Definition (SSB)

G a global symm. group of HThen SSB occurs if in stable TD equilibrium state:

m =:< M >6= 0 (2)

M: observable not G-inv.m: order parameter (of the new phase)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

How can SSB occur?

Problem: There schould not be any SSB!

Why? Averages w.r.t. and = (H)

m =< M >= Tr(M(H)) = 0 (3)

example: Ising model:

m ==1

N ZN

i

i =1

i expH({i}) = 0

(4)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

How can SSB occur?

Solution: take TD limit, but this isnt enough!

one needs: symm. breaking field h extra term: h M

Definition (SSB (Bogolyubov))

limh0

limN

< M >N,h = m 6= 0 (5)

Limits cannot be interchanged!

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Symmetries in PhysicsThe phenomenon of SSB

Remarks

SSB long range order, e.g. in ferro-/antiferro-magnets

SSB phase transitionBut there are phase transitions without SSB, e.g.liquid-vapour: both fully rotational and translationalinvariant!order parameter |l v |

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Goldstone Modes

SSB of continuous symm. new excitations:Goldstone Modes (cost little energy)

example: Spin waves in Heisenberg model

H = J

ij

SiSj = J

ij

cos(ij), Si R3, |Si | = 1

(6)

ferromagnetic phase (SSB) Groundstate: all spins inone direction

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Goldstone Modes

energy cost to rotate one spin: Eg J(1 cos()),(infinitesimal small angle )

energy cost to rotate all spins: Nothing!(due to cont. symm. O(3)) long-wavelength spin-waves

remark: cannot happen with discrete broken symm.!

e.g. Ising model: Eg J (every excitation costs finiteenergy)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Goldstone Theorem

general result proving existence of long-range order

consider correlation functions Gij , e.g. spin-spin corr.~mi : order parameter~hi : symm. breaking field

Gibbs free energy: G{h} = ln(Z )

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Proof of Goldstone Theorem

vertex function: F{m} = G{h}+

i~hi ~mi

(Legendre transformation)

take F G-invariant (in zero external field)

miF{m} = hi

corr. function: Gij = 2G

hi hj

|~hi =0 = 1ij

(Fluctuation-Dissipation Theorem)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Proof of Goldstone Theorem

general property of Legendre transf.: 2Fm2

= (2Gh2

)1

2Fmi m

j

= (G1)ijFourier transformation:[G1(~q)] =

i ei~q(~Ri~Rj ) 2F

mi mj

now: zero field case, uniform order parameter(~mi = const. = ~m, i.e. ~q = 0) and infinitesimaltransformation g = id + t

(Lie group)

t : non-trivial transformation (= transversal modes!)

acting on m g (m) = m + tm m

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Proof of Goldstone Theorem

variation yields: 0 = [ Fmi

] =

j [2F

mi mj

]~mi =~mm

[G1(~q = 0)]t m = 0 (7)

no SSB trivial, since ~m = 0longitudinal transformation trivial, since t m = 0but if ~m 6= 0 detG1(~q = 0) = 0 detG(~q = 0) =

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Goldstone Theorem

Theorem (Goldstone)

If Lie symm. group is spontaneously broken and orderparameter is uniform then the order parameter-orderparameter response function G =< mm > develops a pole.

long range order

appearence of Goldstone Modes

gapless excitation spectrum (zero mass)

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Goldstone Theorem: Remark

In the language of field theory:

partition function: Z =D{}exp[S{}]

with action S{} =

ddxL[(x)],order parameter field (x)

Lagrange density: L = 12

i |i |2 + P[Cn()]

propagators: ij(~x , ~y) = 2{}

i (~x)j (~y)

= ln(Z), i : source term = external field det(1(p = 0)) = 0

Soren Petrat Spontaneous Symmetry Breaking and Goldstone Modes

Spontaneous Symmetry BreakingImplications of SSB

Conclusion

Goldstone ModesGoldstone TheoremMermin-Wagner Theorem

Goldstone Theorem: Remark

remark: order parameter modulated with wave-vector Q

detG1(Q) = 0 (above: special case Q = 0)e.g. liquid-solid phase transition

consider (q) order paramete