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First-principles calculations of oxygen octahedral distortions in LaAlO3/SrTiO3(001) superlattices.

The size, shape and connectivity of oxide octahedra are essential for understanding and controlling the emergent functional properties of ABO3 perovskites. Using first-principles calculations, we systematically studied the oxygen octahedral rotation and deformation in LaAlO3/SrTiO3(001) superlattices. Superlattices with electron- or hole-doped interfaces, or both, are compared. The results showed that there are at least three different types of oxygen octahedral distortions in these superlattices, which is more than what had previously been reported in the literature. We demonstrate that interfacial oxygen octahedral coupling and hole-doping, in addition to epitaxial strain, are the key factors underlying the formation of multiple types of oxygen octahedral rotations in these systems. We confirm that oxygen octahedral rotations and deformations play an essential role in insulator-metal transitions. Furthermore, octahedral distortion leads to ferroelectricity like dipole formation with the polarization vector always pointing to the positively charged interfaces.