Suppression and revival of long-range ferromagnetic order in the multiorbital Fermi-Hubbard model

TL;DR

This study uses dynamical mean-field theory to show how including full SU(2) symmetry in the multiorbital Fermi-Hubbard model suppresses ferromagnetic order, which can be revived with increasing orbital number, linking to classical spin behavior.

Contribution

It demonstrates the significant impact of spin-rotational symmetry on ferromagnetic order in multiorbital models, extending understanding beyond density-density interaction approximations.

Findings

Suppression of ferromagnetism is strongest in two-orbital models.

Magnetic order can revive with three or more orbitals.

Spin-flip processes are crucial in the double-exchange context.

Abstract

By means of dynamical mean-field theory allowing for complete account of SU(2) rotational symmetry of interactions between spin-1/2 particles, we observe a strong effect of suppression of ferromagnetic order in the multiorbital Fermi-Hubbard model in comparison with a widely used restriction to density-density interactions. In the case of orbital degeneracy, we show that the suppression effect is the strongest in the two-orbital model (with effective spin $S_{\rm eff}=1$) and significantly decreases when considering three orbitals ($S_{\rm eff}=3/2$), thus magnetic ordering can effectively revive for the same range of parameters, in agreement with arguments based on vanishing of quantum fluctuations in the limit of classical spins ($S_{\rm eff}\to\infty$). We analyze a connection to the double-exchange model and observe high importance of spin-flip processes there as well.