Strain-modulated anisotropic electronic structure in superconducting RuO2 films

The binary ruthenate, ${\mathrm{RuO}}_{2}$, 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) ${\mathrm{RuO}}_{2}$ 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 $[110]/[1\overline{1}0]$ 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. These results underscore the importance of the effective reduction from tetragonal to orthorhombic lattice symmetry in (110) ${\mathrm{RuO}}_{2}$ films and its relevance towards the superconducting and magnetic properties.