Orbital reflectometry of oxide heterostructures

The occupation of electronic orbitals on the surface and interface of oxide thin films and heterostructures is a key influence over their properties, including magnetism and superconductivity. A new spectroscopy technique now provides the first quantitative, spatially resolved data of orbital occupation in oxide structures. The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties 1 . Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties 2 , 3 , 4 , 5 , 6 , 7 , 8 , but could not thus far be probed in a quantitative manner 9 , 10 , 11 . Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ∼3% in the occupation of Ni e g orbitals in adjacent atomic layers of a LaNiO 3 –LaAlO 3 superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.