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

Abstract

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.