Renormalization group flows into phases with broken symmetry

TL;DR

This paper introduces a novel fermionic renormalization group method that incorporates symmetry breaking directly into the flow, avoiding the need for auxiliary fields, and accurately captures spontaneous symmetry breaking phenomena.

Contribution

It presents a new approach to extend fermionic RG flows into symmetry-broken phases without Hubbard-Stratonovich decoupling, using an initial infinitesimal symmetry-breaking term.

Findings

Flow saturates at a nonzero value indicating symmetry breaking

Method reproduces exact solution for the reduced BCS model

Discusses emergence of Goldstone bosons in the flow

Abstract

We describe a way to continue the fermionic renormalization group flow into phases with broken global symmetry. The method does not require a Hubbard-Stratonovich decoupling of the interaction. Instead an infinitesimally small symmetry-breaking component is inserted in the initial action, as an initial condition for the flow of the selfenergy. Its flow is driven by the interaction and at low scales it saturates at a nonzero value if there is a tendency for spontaneous symmetry breaking in the corresponding channel. For the reduced BCS model we show how a small initial gap amplitude flows to the value given by the exact solution of the model. We also discuss the emergence of the Goldstone boson in this approach.