Valence-state reflectometry of complex oxide heterointerfaces

Emergent phenomena in transition-metal-oxide heterostructures such as interface superconductivity and magnetism have been attributed to electronic reconstruction, which, however, is difficult to detect and characterise. Here we overcome the associated difficulties to simultaneously address the electronic degrees of freedom and distinguish interface from bulk effects by implementing a novel approach to resonant X-ray reflectivity (RXR). Our RXR study of the chemical and valance profiles along the polar (001) direction of a LaCoO₃ film on NdGaO₃ reveals a pronounced valence-state reconstruction from Co³⁺ in the bulk to Co²⁺ at the surface, with an areal density close to 0.5 Co²⁺ ions per unit cell. An identical film capped with polar (001) LaAlO₃ maintains the Co³⁺ valence over its entire thickness. We interpret this as evidence for electronic reconstruction in the uncapped film, involving the transfer of 0.5e⁻ per unit cell to the subsurface CoO₂ layer at its LaO-terminated polar surface.