Symmetry selective oxygen orbital polarization in spinel Al₂O₃ films

Orbital polarization is common for the 3d orbitals of cations as a result of strain, symmetry breaking, or charge transfer, but very rare for 2p orbitals of simple anions. Here, we report a symmetry-selective measurement of orbital polarization for O 2p orbitals in γ-Al₂O₃/SrTiO₃ thin films via linearly polarized X-ray absorption spectroscopy (XAS) at the O K-edge. The O 2p orbitals with octahedral coordination exhibit a large subband splitting exceeding ~ 0.35 eV, while those with tetrahedral coordination show a very weak orbital polarization. These results pave a new way to engineer band structure and quantum states of functional oxide materials via local symmetry effects on anion orbitals.