Electronic correlations, magnetism and Hund's rule coupling in the ruthenium perovskites SrRuO$_3$ and CaRuO$_3$

TL;DR

This study uses density functional plus dynamical mean field theory to analyze the electronic correlations, magnetism, and Hund's rule effects in SrRuO$_3$ and CaRuO$_3$, revealing their placement as Hund's metals and the influence of structural distortions.

Contribution

It provides detailed phase diagrams and distinguishes Mott and Hund's dominated regimes, elucidating the role of structural distortions and Hund's coupling in ruthenate materials.

Findings

Materials are Hund's metals with strong Hund's coupling influence.

Magnetism in SrRuO$_3$ is explained by density of states effects.

Structural distortions impact phase boundaries and magnetic properties.

Abstract

A comparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials CaRuO$_3$ and SrRuO$_3$ is presented. Phase diagrams are determined for both materials as a function of Hubbard repulsion $U$ and Hund's rule coupling $J$. Metallic and insulating phases are found, as are ferromagnetic and paramagnetic states. The locations of the relevant phase boundaries are determined. Based on the computed phase diagrams, Mott-dominated and Hund's dominated regimes of strong correlation are distinguished. Comparison of calculated properties to experiments indicates that the actual materials are in the Hund's coupling dominated region of the phase diagram so can be characterized as Hund's metals, in common with other members of the ruthenate family. Comparison of the phase diagrams for the two materials reveals the role played by rotational and tilt (GdFeO$_3$-type) distortions of the ideal perovskite structure. The presence of magnetism in SrRuO$_3$ and its absence in CaRuO$_3$ despite the larger mass and larger tilt/rotational distortion amplitude of CaRuO$_3$ can be understood in terms of density of states effects in the presence of strong Hund's coupling. Comparison of the calculated low-$T$ properties of CaRuO$_3$ to those of SrRuO$_3$ provides insight into the effects of magnetic order on the properties of a Hund's metal. The study provides a simultaneous description of magnetism and correlations and explicates the roles played by band theory and Hubbard and Hund's interactions.