Links between La $$N_{4,5}$$-edge RIXS probed A-site covalency, structural parameters and superconducting properties in optimally doped La$$_{1.84}$$M$$_{0.16}$$CuO$$_4$$ (M = Ca, Sr, Ba) epitaxial films and the La$$_{2}$$CuO$$_4$$ host lattice

Abstract We investigate La-site bonding by La N $$_{4,5}$$ -edge RIXS together with structural parameters and the superconducting transition temperature $$T_c$$ of optimally doped La $$_{1.84}$$ M $$_{0.16}$$ CuO $$_4$$ (M = Ca, Sr, Ba) epitaxial films and the La $$_2$$ CuO $$_4$$ host lattice. La and M in the A-site have been considered to be part of the axial hybrid state formed by Cu 4 s , apical O 2 $$p_z$$ , and rare earth (La) states. In La $$_{2-x}$$ M $$_{x}$$ CuO $$_4$$ (M = Ca, Sr, Ba) these states have been proposed as contributing to the superconducting transition temperature $$T_c$$ . We observe a systematic decrease of the La $$5p^54f^1$$ $$^3D_1$$ RIXS energy loss across the Ca–Sr–Ba doped film series. Based on previous La N $$_{4,5}$$ -edge RIXS studies, this trend is consistent with a La-site response to enhanced hybridization and local screening. The trend follows increasing A-site ionic radius and c -axis expansion, but these quantities are coupled to tetragonal distortion, unit-cell volume, epitaxial strain, relaxation, and possible symmetry changes. The undoped LCO host lattice shows a La $$^3D_1$$ energy close to the Sr-doped film, suggesting that its La-site environment cannot be described by ionic radius alone. However, because the LCO film differs in thickness and relaxation state from the doped films, this comparison is treated as qualitative. The superconducting transition temperature does not follow the La $$^3D_1$$ trend monotonically: the Sr-doped film has the highest $$T_c$$ , whereas the Ba-doped film shows a reduced $$T_c$$ despite the lowest $$^3D_1$$ energy loss. This indicates that the local A-site bonding response probed by La N $$_{4,5}$$ -edge RIXS is only one contribution among broader structural and electronic effects.