Improved conduction and orbital polarization in ultrathin LaNiO₃ sublayer by modulating 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 approach to stabilize a distinct oxygen octahedron rotation (OOR) characterized by <math xmlns="http://www.w3.org/1998/Math/MathML"> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>c</mi></mrow> <mrow><mo>+</mo></mrow> </msup> </math> in the ultrathin LaNiO₃ sublayers of the LaNiO₃/CaTiO₃ superlattices. Unlike the <math xmlns="http://www.w3.org/1998/Math/MathML"> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>c</mi></mrow> <mrow><mo>-</mo></mrow> </msup> </math> OOR in the LaNiO₃ bare film, the <math xmlns="http://www.w3.org/1998/Math/MathML"> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>c</mi></mrow> <mrow><mo>+</mo></mrow> </msup> </math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <msub><mrow><mi>d</mi></mrow> <mrow> <msup><mrow><mi>x</mi></mrow> <mrow><mn>2</mn></mrow> </msup> <mo>-</mo> <msup><mrow><mi>y</mi></mrow> <mrow><mn>2</mn></mrow> </msup> </mrow> </msub> </math> 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 <math xmlns="http://www.w3.org/1998/Math/MathML"> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>a</mi></mrow> <mrow><mo>-</mo></mrow> </msup> <msup><mrow><mi>c</mi></mrow> <mrow><mo>+</mo></mrow> </msup> </math> 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.