The Mermin-Wagner Theorem - Condensed Matter Theory Group

How symmetry breaking occurs in principle
Actors
Proof of the Mermin-Wagner Theorem
Discussion
It turns out that such averages may be unstable under an
infinitesimal perturbation of the Hamiltonian
one can define the quasi-average:
Hν = H + νH ′ − µ ˆN
〈A〉 q = lim
ν→0 lim
V →∞ tr
e −βHν
A
The quasi-average does not need to coincide with the normal
average
i
C
= lim q ν→0 tr
e −βHν H, Γ i
S = 0 −
Andreas Werner The Mermin-Wagner Theorem