Spontaneous Symmetry Breaking and Goldstone Modes Symmetry Breaking Implications of SSB Conclusion Spontaneous Symmetry Breaking and Goldstone Modes S oren Petrat May 26, 2009 S

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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