Asymmetric Orbital-Lattice Interactions in Ultrathin Correlated Oxide Films

Abstract

Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.

Electronic structure and orbital polarization of LaNiO3with a reduced coordination and under strain: A first-principles study

“…It is found from XLD measurement that compressively strained LNO thin film has negative orbital polarization whereas the tensile strain does not produce the sign reversal in the polarization 17 , which is in a contradiction to the simple electrostatic Madelung picture assuming the same energy levels both for d x 2 −y 2 and d 3z 2 −r 2 17 . Our calculation provides a natural explanation as a surface effect dominant situation; i.e., the electronic structure change mainly caused by the reduced coordination is responsible for the negative polarization, and the effect persists even under the tensile strain.…”

“…As a result, thin film LNO and LNO-based heterostructures have been studied under various strained conditions to control the correlation strength and the metalinsulator phase transition [11][12][13][14] in an effort to find possible high temperature superconductivity 15 and conductivity enhancement 16 . Strain affects most strongly the orbital character of the nickelates 15,[17][18][19][20] . Recently, several papers have focused on how changes in orbital polarization in heterostructures and thin films affect the electronic structure and thereby the macroscopic material properties 15,[17][18][19][20][21][22] .…”

“…Strain affects most strongly the orbital character of the nickelates 15,[17][18][19][20] . Recently, several papers have focused on how changes in orbital polarization in heterostructures and thin films affect the electronic structure and thereby the macroscopic material properties 15,[17][18][19][20][21][22] .…”

“…Beside the transformation into a different structural phase 17 , the polarization change under compressive and tensile strain has mainly been analyzed using Madelung energies. However, the detailed changes in electronic structure cannot be understood from such a simple electrostatic approach 17,18 . For example, both in film and in heterostructures of LNO, compressive strain make the d 3z 2 −r 2 orbital more occupied, as one would expect.…”

“…Vacuum layer thickness is of 7.0Å which is large enough to make the basis function overlap negligible. While the polar surface may not be well stabilized in general, these instability can be compensated by the large covalency in the nickelates 11,12,17,18 . The geometry relaxation has been performed with the force criterion of 10 −3 Hartree/Bohr.…”