Magnetic transitions in strong coupling expansions for nearly degenerate states

TL;DR

This paper develops a strong coupling expansion method for a two-band Hubbard model with nearly degenerate states, accurately capturing magnetic phase transitions at large Coulomb interactions.

Contribution

It introduces a fourth order perturbation approach based on a four-dimensional low-energy subspace for systems with nearly degenerate states, improving accuracy in predicting magnetic transitions.

Findings

Accurately describes the transition from antiferromagnetic to ferromagnetic ground state.

Demonstrates the effectiveness of a four-dimensional low-energy subspace approach.

Provides a comparative analysis of perturbation schemes for nearly degenerate systems.

Abstract

We consider a strong coupling expansion for a two-band Hubbard model on two sites with nearly degenerate states. A comparative analysis is performed for different schemes of perturbation theory which are applicable to systems with nearly degenerate states. A fourth order approach which builds on a four-dimensional low-energy subspace with nearly degenerate states captures accurately the transition from an antiferromagnetic to a ferromagnetic ground state at large on-site Coulomb interaction.