Tailoring the low-energy physics of RuO₂by epitaxial strain

Rutile ruthenium dioxide (RuO₂) was long presumed to be a paramagnetic metal with weak electronic correlations, until recent measurements of antiferromagnetism in bulk single crystals by Berlijn et al. Motivated by this discovery to better understand its electronic structure, we synthesized RuO₂ thin films by molecular-beam epitaxy on rutile TiO₂ substrates and characterized these films using in situ angle-resolved photoemission spectroscopy (ARPES). Comparing our ARPES data with density-functional calculations, we find that: (1.) electron-phonon coupling accounts for most of the modest quasiparticle mass renormalizations observed in RuO₂, and (2.) a sizable crystal field splitting in the rutile structure lifts the threefold degeneracy of the t_2g manifold spanning the Fermi level (E_F), causing strong departures from the prototypical Hund’s metal behavior observed in perovskite-based ruthenates, despite having the same electron count of 4d⁴. Guided by this understanding of the effective low-energy physics, we explain how epitaxial strain modifies the orbital occupations in RuO₂ films grown on different orientations of TiO₂ substrates, and discuss how concomitant changes to the density of states near E_F feed back into the instability of strained RuO₂ towards superconductivity.