Strain-modulated anisotropic electronic structure in superconducting RuO2 films

The binary ruthenate, RuO₂, has been the subject of intense interest due to its itinerant antiferromagnetism and strain-induced superconductivity. The strain mechanism and its effect on the microscopic electronic states leading to the normal and superconducting state, however, remain undisclosed. Here, we investigate highly strained epitaxial (110) RuO₂ films using polarization-dependent oxygen K-edge x-ray absorption spectroscopy (XAS). Through the detection of pre-edge peaks arising from O:2p - Ru:4d hybridization, we uncover the effects of epitaxial strain on the orbital/electronic structure near the Fermi level. Our data show robust strain-induced shifts of orbital levels and a reduction of hybridization strength. Furthermore, we reveal a pronounced in-plane anisotropy of the electronic structure along the [110] / [1$\\bar{1}$0] directions naturally stemming from the symmetry-breaking epitaxial strain of the substrate. The B_2g symmetry component of the epitaxially enforced strain breaks a sublattice degeneracy, resulting in an increase of the density of states at the Fermi level (E_F), possibly paving the way to superconductivity. Finally, these results underscore the importance of the effective reduction from tetragonal to orthorhombic lattice symmetry in (110) RuO₂ films and its relevance towards the superconducting and magnetic properties.