The Mermin-Wagner Theorem - Condensed Matter Theory Group

How symmetry breaking occurs in principle
Actors
Proof of the Mermin-Wagner Theorem
Discussion
For systems in statistical equilibrium the expectation value of an
operator A is given by
e −βH
A
〈A〉 = lim
V →∞ tr
If the Hamiltonian displays a continuous symmetry S it commutes
with the generators Γi S of the corresponding symmetry group
i
H, ΓS = 0
If some operator is not invariant under the transformations of S,
i i
B, ΓS = C = 0
the average of the commutator C i vanishes:
C i = 0
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Andreas Werner The Mermin-Wagner Theorem