The dynamical vertex approximation for many-electron systems with spontaneously broken SU(2)-symmetry

TL;DR

This paper extends the dynamical vertex approximation (DΓA) to magnetically ordered phases, providing a framework to study broken SU(2)-symmetry in many-electron systems, with applications to the Hubbard model.

Contribution

The paper develops a generalized DΓA formalism for magnetic phases, including algorithmic simplifications and explicit expressions for the self-energy in ferromagnetic and antiferromagnetic states.

Findings

Captures key features of metallic and insulating antiferromagnets

Provides a reliable approach for studying magnetic order in correlated systems

Illustrates the physics of antiferromagnetic ground states with static mean-field inputs

Abstract

We generalize the formalism of the dynamical vertex approximation (D$\Gamma$A) -- a diagrammatic extension of the dynamical mean-field theory (DMFT)-- to treat magnetically ordered phases. To this aim, we start by concisely illustrating the many-electron formalism for performing ladder resummations of Feynman diagrams in systems with broken SU(2)-symmetry, associated to ferromagnetic (FM) or antiferromagnetic (AF) order. We then analyze the algorithmic simplifications introduced by taking the local approximation of the two-particle irreducible vertex functions in the Bethe-Salpeter equations, which defines the ladder implementation of D$\Gamma$A for magnetic systems. The relation of this assumption with the DMFT limit of large coordination-number/ high-dimensions is explicitly discussed. As a last step, we derive the expression for the ladder D$\Gamma$A self-energy in the FM- and AF-ordered phases of the Hubbard model. The physics emerging in the AF-ordered case is explicitly illustrated by means of approximated calculations based on a static mean-field input for the D$\Gamma$A equations. The results obtained capture fundamental aspects of both metallic and insulating ground states of two-dimensional antiferromagnets, providing a reliable compass for future, more extensive applications of our approach. Possible routes to further develop diagrammatic-based treatments of magnetic phases in correlated electron systems are briefly outlined in the conclusions.