Electronic properties of nickelate superconductor R3Ni2O7 with oxygen vacancies

The discovery of superconductivity in La3Ni2O7 has attracted significant research interest in the field of nickelate superconductors. Despite extensive studies on pristine La3Ni2O7, the impact of oxygen vacancies (VO), a common type of intrinsic defect in oxides, on electronic structures and superconductivity in La3Ni2O7 remains unclear. In this article, we identify the most energetically favorable location for VO formation as the oxygen atom connecting the NiO6 bilayer, resulting in a significant reduction in the lattice constant along the c-axis. Interestingly, the electronic structure undergoes notable changes, particularly for the Ni dz2 and Ni dx2-y2 orbitals. The Ni dz2 orbitals change from partially filled in the pristine La3Ni2O7 to completely filled in the presence of VO, leading to a considerable decrease of its proportion near the Fermi level. Conversely, the proportion of Ni dx2-y2 states increases due to the orbital localization and slight upward shift. Additionally, we observe a significant increase in the hopping of intra-bilayer Ni dz2 orbitals when the VO exists, but with an opposite sign, which differs greatly from the previous understanding. The inter-orbital hopping between Ni dz2 and Ni dx2-y2 orbitals also changes its sign in the presence of VO. Our results indicate that the formation of VO may be harmful to the superconductivity in La3Ni2O7, given the general assumption for the critical role of Ni dz2 in generating superconductivity. Furthermore, we suggest that Ce3Ni2O7, which shares similar electronic structures to La3Ni2O7 but has a larger lattice volume, may be a better candidate for nickelate superconductor due to its lower VO concentration.