Improved conduction and orbital polarization in ultrathin LaNiO sublayer by modulating octahedron rotation in LaNiO/CaTiO 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 in the ultrathin LaNiO sublayers of the LaNiO/CaTiO superlattices. Unlike the OOR in the LaNiO bare film, the OOR favors high conductivity, driving the LaNiO 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 orbital is achieved in LaNiO sublayers. The largest change of occupancy is as high as 35%, observed in the 2 u.c.-thick LaNiO sublayers sandwiched between 4 u.c.-thick CaTiO sublayers. X-ray absorption spectra indicate that the OOR pattern of LaNiO achieved in the LaNiO/CaTiO heterostructures has significantly enhanced the Ni-3d/O-2p hybridization, stabilizing the metallic phase in ultrathin LaNiO 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.