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.