Electronic correlations, magnetism, and Hund's rule coupling in the ruthenium perovskites SrRuO3 and CaRuO3

Acomparative density functional plus dynamical mean field theory study of the pseudocubic ruthenate materials CaRuO3 and SrRuO3 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 (GdFeO3-type) distortions of the ideal perovskite structure. The presence of magnetism in SrRuO3 and its absence in CaRuO3 despite the larger mass and larger tilt/rotational distortion amplitude of CaRuO3 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 CaRuO3 to those of SrRuO3 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.