Competition of crystal field splitting and Hund's rule coupling in two-orbital magnetic metal-insulator transitions

TL;DR

This paper investigates how crystal field splitting and Hund's rule coupling influence magnetic metal-insulator transitions in a two-orbital Hubbard model, revealing their competing effects on various insulating and metallic phases.

Contribution

It introduces a multi-orbital slave-boson mean field approach to analyze the interplay between crystal field splitting and Hund's coupling in metal-insulator transitions.

Findings

Large crystal field splitting stabilizes the orbital insulator.

Strong Hund's coupling suppresses the orbital insulator and promotes high-spin states.

Magnetic correlations shift the orbital insulator transition to lower Coulomb interaction U.

Abstract

Competition of crystal field splitting and Hund's rule coupling in magnetic metal-insulator transitions of half-filled two-orbital Hubbard model is investigated by multi-orbital slave-boson mean field theory. We show that with the increase of Coulomb correlation, the system firstly transits from a paramagnetic (PM) metal to a {\it N\'{e}el} antiferromagnetic (AFM) Mott insulator, or a nonmagnetic orbital insulator, depending on the competition of crystal field splitting and the Hund's rule coupling. The different AFM Mott insulator, PM metal and orbital insulating phase are none, partially and fully orbital polarized, respectively. For a small $J_{H}$ and a finite crystal field, the orbital insulator is robust. Although the system is nonmagnetic, the phase boundary of the orbital insulator transition obviously shifts to the small $U$ regime after the magnetic correlations is taken into account. These results demonstrate that large crystal field splitting favors the formation of the orbital insulating phase, while large Hund's rule coupling tends to destroy it, driving the low-spin to high-spin transition.