An <i>ab initio</i> study of electron-hole pairs in the insulating single layer cuprates and nickelates

Advanced density functionals have proven to be an indispensable tool in studying correlated materials exhibiting the delicate interplay between complex charge and non-collinear magnetic order. We perform a comparison study of the electronic and magnetic structure of a prototypical high-temperature superconductor La₂CuO₄ with the isostructural single-layer nickelate La₂NiO₄. We find key differences in the low-energy electronic structure, where in particular, La₂NiO₄ exhibits an enhanced Hund's coupling. Additionally, utilizing our accurate ab initio ground state, we obtain the excitonic dispersion for each material. Excitons in La₂CuO₄ are delocalized and can freely move in the CuO₂ plane without disturbing the antiferromagnetic order. In contrast, in La₂NiO₄ we find the low-lying excitonic states to be extremely localized, producing a nearly flat dispersion. Our results are in excellent agreement with RIXS observations, and give insights into the excited state dynamics of the cuprates and nickelates. Finally, we will briefly connect our results to the newly discovered superconducting infinite-layer nickelates.