Metallic Conduction and Large Orbital Polarization in Ultrathin LaNiO3 Sublayer Achieved by Modulating Oxygen Octahedron Rotation in LaNiO3/CaTiO3 Superlattices

Abstract Artificial oxide heterostructures have provided promising platforms for the exploration of emergent quantum phases with extraordinary properties. Here, we demonstrate an effective approach to stabilize a distinct oxygen octahedron rotation (OOR) characterized by a⁻b⁻c⁺ in the ultrathin LaNiO₃ sublayers of the LaNiO₃/CaTiO₃ superlattices. Unlike the a⁻b⁻c⁻ OOR in the LaNiO₃ bare film, the a⁻b⁻c⁺ OOR favors high conductivity, driving the LaNiO₃ sublayer to metallic state even when the layer thickness is as thin as 2 unit cells (u.c.). Simultaneously, strongly preferred occupation of dx₂-y₂ orbital is achieved in LaNiO₃ sublayers. The largest change of occupancy is as high as 35%, observed in the 2 u.c.-thick LaNiO₃ sublayers sandwiched between 4 u.c.-thick CaTiO₃ sublayers. X-ray absorption spectra indicate that the a⁻b⁻c⁺ OOR pattern of LaNiO₃ achieved in the LaNiO₃/CaTiO₃ heterostructures has significantly enhanced the Ni-3d/O-2p hybridization, stabilizing the metallic phase in ultrathin LaNiO₃ sublayers. The present work demonstrates that modulating the mode of OOR through heteroepitaxial synthesis can modify the orbital-lattice correlations in correlated perovskite oxides, revealing hidden properties of the materials.