Strain-induced orbital-energy shift in antiferromagnetic RuO2 revealed by resonant elastic x-ray scattering

In its ground state, RuO₂ was long thought to be an ordinary metallic paramagnet. Recent neutron and x-ray diffraction revealed that bulk RuO₂ is an antiferromagnet with T_N above 300 K. Furthermore, epitaxial strain induces superconductivity in thin films of RuO₂ below 2 K. Here, we present a resonant elastic x-ray scattering study at the Ru L₂ edge of the strained RuO₂ films exhibiting the strain-induced superconductivity. Here, we observe an azimuthal modulation of the 100 Bragg peak consistent with bulk. Most notably, in the strained films displaying superconductivity, we observe a ~ 1 eV shift of the Ru e_g orbitals to a higher energy. The energy shift is smaller in thicker, relaxed films and films with a different strain direction. Our results provide further evidence of the utility of epitaxial strain as a tuning parameter in complex oxides.