Understanding the Transition from Metal to Hund's Insulator in CaFeO$_3$

Strongly correlated materials show great potential for energy-efficient transistors thanks to the sharp metal-insulator transition (MIT) that some of them exhibit. However, this MIT is almost always coupled to a structural distortion, which can lead to fatigue effects and limit the switching frequency. To better understand the different types of distortions, more fundamental knowledge about the complicated physics at work here is needed.<br/><br/>Recently, materials where strong correlations are caused by the Hund's interaction<b> </b>have attracted a lot of attention. In some cases, a dominant Hund's interaction can even<b> </b>lead to the emergence of a charge-disproportionated insulating (CDI) or "Hund's insulating" state, which can cause an MIT.<b> </b>One example is<b> </b>the perovskite transition-metal oxide CaFeO$_3$ (CFO) with a transition close to room temperature. This transition couples to a structural<b> </b>distortion that creates alternating large and small FeO$_6$ octahedra, leading to two inequivalent Fe sites with<b> </b>nominal Fe$^{5+}$ and Fe$^{3+}$ charge states.<br/><br/>We study CFO<b> </b>using density functional theory (DFT) and<b> </b>dynamical mean-field theory (DMFT). To<b> </b>characterize the CDI state, we first apply DMFT to a five-orbital Hubbard model applicable to CFO and demonstrate the emergence of the CDI phase [1]. We then investigate the energetics of the transition<b> </b>using fully self-consistent DFT+DMFT calculations, which show that both structural and electronic properties of the CDI state can be well described within DFT+DMFT.<br/>Finally, we discuss the ligand-hole character of the charge disproportionation and analyze the role of the zero or even negative charge-transfer energy in CFO as well as its influence on the emergence of the MIT.<br/><br/>[1] M. E. Merkel and C. Ederer, <i>Phys. Rev. B</i><b> </b>104, 165135 (2021), doi: 10.1103/PhysRevB.104.165135