Local electronic structure of rutile RuO$_2$

TL;DR

This study investigates the electronic structure of rutile RuO$_2$ using advanced x-ray techniques, revealing the dominant role of crystal field effects over spin-orbit coupling and emphasizing the importance of local symmetry in its electronic properties.

Contribution

The paper provides a detailed analysis of the electronic structure of RuO$_2$, highlighting the significant crystal field splitting and its impact on the role of spin-orbit coupling, extending understanding to other rutile compounds.

Findings

Crystal field splitting exceeds spin-orbit coupling energy.

Local symmetry significantly influences electronic states.

Analysis applicable to other rutile materials.

Abstract

Recently, rutile RuO$_2$ has raised interest for its itinerant antiferromagnetism, crystal Hall effect, and strain-induced superconductivity. Understanding and manipulating these properties demands resolving the electronic structure and the relative roles of the rutile crystal field and $4d$ spin-orbit coupling (SOC). Here, we use O-K and Ru $M_3$ x-ray absorption (XAS) and Ru $M_3$ resonant inelastic x-ray scattering (RIXS) to disentangle the contributions of crystal field, SOC, and electronic correlations in RuO$_2$. The locally orthorhombic site symmetry of the Ru ions introduces significant crystal field contributions beyond the approximate octahedral coordination yielding a crystal field energy scale of $\Delta(t_{2g})\approx 1$ eV breaking the degeneracy of the $t_{2g}$ orbitals. This splitting exceeds the Ru SOC ($\approx160$ meV) suggesting a more subtle role of SOC, primarily through the modification of itinerant (rather than local) $4d$ electronic states, ultimately highlighting the importance of the local symmetry in RuO$_2$. Remarkably, our analysis can be extended to other members of the rutile family, thus advancing the comprehension of the interplay among crystal field symmetry, electron correlations, and SOC in transition metal compounds with the rutile structure.