Fermionic Renormalization Group Flow at All Scales: Breaking a Discrete Symmetry

We extend the functional renormalization group technique in a modi cation of the one‐particle irreducible scheme to study discrete symmetry breaking at nite temperature. As an instructive example, we employ the technique to access both the symmetric and the symmetry‐broken phase of a charge‐density wave mean‐ eld model. We study the half‐ lled case, and thus the breaking of a discrete symmetry, at nite temperature. A small external symmetry‐breaking eld allows us to access the symmetry‐broken state without encountering any divergence in the o w. We show diagrammatically that our method is equivalent to an exact resummation treatment. We numerically study the dependence of the o w on the external eld and on temperature.