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

Abstract

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 ${L}_{2,3}$ edge of a tetragonal $\mathrm{CuO}/{\mathrm{SrTiO}}_{3}$ heterostructure. Using first-principles calculations, we propose a scenario that explains both types of moments, based on the formation of oxygen vacancies in the ${\mathrm{TiO}}_{2}$ 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 ${L}_{2,3}$ edge since the valence of the Ti atoms remains unchanged. Strong structural distortions breaking both the local and global fourfold rotation ${C}_{4}$ 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.