Charge disproportionation and Hund's insulating behavior in a five-orbital Hubbard model applicable to $d^4$ perovskites

TL;DR

This study investigates the phase transitions and insulating states in a five-orbital Hubbard model relevant to $d^4$ perovskites, revealing the existence of a Hund's insulator and its characteristics through dynamical mean-field theory.

Contribution

It introduces a comprehensive phase diagram for a five-orbital model, highlighting the Hund's insulator and comparing simplified models to the full orbital description.

Findings

Identification of charge-disproportionated insulating phase

Characterization of Hund's insulator as a distinct phase

Validation of simplified models for high-spin regions

Abstract

We explore the transition to a charge-disproportionated insulating phase in a five-orbital cubic tight-binding model applicable to transition-metal perovskites with a formal $d^4$ occupation of the transition-metal cation, such as ferrates or manganites. We use dynamical mean-field theory to obtain the phase diagram as a function of the average local Coulomb repulsion $U$ and the Hund's coupling $J$. The main structure of the phase diagram follows from the zero band-width (atomic) limit and represents the competition between high-spin and low-spin homogeneous and an inhomogeneous charge-disproportionated state. This results in two distinct insulating phases: the standard homogeneous Mott insulator and the inhomogeneous charge-disproportionated insulator, recently also termed Hund's insulator. We characterize the unconventional nature of this Hund's insulating state. Our results are consistent with previous studies of two- and three-orbital models applicable to isolated $t_{2g}$ and $e_g$ subshells, respectively, with the added complexity of the low-spin/high-spin transition. We also test the applicability of an effective two-orbital ($e_g$-only) model with disordered $S=3/2$ $t_{2g}$ core spins. Our results show that the overall features of the phase diagram in the high-spin region are well described by this simplified two-orbital model but also that the spectral features exhibit pronounced differences compared to the full five-orbital description.