Reduced effective spin-orbital degeneracy and spin-orbital ordering in paramagnetic transition metal oxides: Sr2IrO4 vs. Sr2RhO4

TL;DR

This paper investigates how spin-orbit interactions and structural factors influence electronic states in Sr2IrO4 and Sr2RhO4, revealing spin-orbital ordering in the former and reduced degeneracy in the latter, with implications for their electronic properties.

Contribution

It extends density functional and dynamical mean field theory to materials with strong spin-orbit coupling, providing insights into spin-orbital effects in transition metal oxides.

Findings

Sr2IrO4 is a spin-orbitally ordered Mott insulator at room temperature.

Sr2RhO4 remains metallic with reduced spin-orbital degeneracy.

Theoretical spectra agree with experimental photoemission data for Sr2RhO4.

Abstract

We discuss the notions of spin-orbital polarization and ordering in paramagnetic materials, and address their consequences in transition metal oxides. Extending the combined density functional and dynamical mean field theory scheme to the case of materials with large spin-orbit interactions, we investigate the electronic excitations of the paramagnetic phases of Sr2IrO4 and Sr2RhO4. We show that the interplay of spin-orbit interactions, structural distortions and Coulomb interactions suppresses spin-orbital fluctuations. As a result, the room temperature phase of Sr2IrO4 is a paramagnetic spin-orbitally ordered Mott insulator. In Sr2RhO4, the effective spin-orbital degeneracy is reduced, but the material remains metallic, due to both, smaller spin-orbit and smaller Coulomb interactions. We find excellent agreement of our ab-initio calculations for Sr2RhO4 with angle-resolved photoemission, and make predictions for spectra of the paramagnetic phase of Sr2IrO4.