Oxygen vacancies at the origin of pinned moments in oxide interfaces: The example of tetragonal CuO/SrTiO3

Obtaining an accurate theoretical description of the emergent phenomena in oxide heterostructures is a major challenge. Recently, intriguing paramagnetic spin and pinned orbital moments have been discovered by x-ray magnetic circular dichroism measurements at the Cu L2,3 edge of a tetragonal CuO/SrTiO3 heterostructure. Using first-principles calculations, we propose a scenario that explains both types of moments, based on the formation of oxygen vacancies in the TiO2 interface layer. We show the emergence of a paramagnetic two-dimensional electron gas hosted in the interface CuO layer. It is invisible at the Ti L2,3 edge since the valence of the Ti atoms remains unchanged. Strong structural distortions breaking both the local and global fourfold rotation C4 symmetries at the interface lead to the in-plane pinning of the Cu orbital moment close to the vacancy. Our results, and in particular the pinning of the orbital moment, may have implications for other systems, especially monoxide/dioxide interfaces with similar metal-oxygen bond length and weak spin-orbit coupling.