Improved conduction and orbital polarization in ultrathin LaNiO3 sublayer by modulating octahedron rotation in LaNiO3/CaTiO3 superlattices

Artificial oxide heterostructures have provided promising platforms for the exploration of emergent quantum phases with extraordinary properties. Here, we demonstrate an approach to stabilize a distinct oxygen octahedron rotation (OOR) characterized by $${a}^{-}{a}^{-}{c}^{+}$$ a − a − c + in the ultrathin LaNiO3 sublayers of the LaNiO3/CaTiO3 superlattices. Unlike the $${a}^{-}{a}^{-}{c}^{-}$$ a − a − c − OOR in the LaNiO3 bare film, the $${a}^{-}{a}^{-}{c}^{+}$$ a − a − c + OOR favors high conductivity, driving the LaNiO3 sublayer to a metallic state of ~100 K even when the layer thickness is as thin as 2 unit cells (u.c.). Simultaneously, strongly preferred occupation of $${d}_{{x}^{2}-{y}^{2}}$$ d x 2 − y 2 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 $${a}^{-}{a}^{-}{c}^{+}$$ a − a − 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.