| Oxide heterostructures have been attracting great attention due to extraordinary phenomena occurring at the interface and their potential application for device design employing oxide materials. A particularly fascinating system is the two-dimensional conductive interface between the band insulators LaAlO3 (LAO) and SrTiO3 (STO), which can be even driven to magnetic and superconducting phases at low temperatures. Resonant inelastic x-ray scattering at Ti L-edges is particularly suitable to address the electronic structure of its interface since the conducting Ti3+ carriers clearly display the signature of strong dd excitations while Ti4+ states exhibit only elastic excitations in the low energy loss regime. Our studies on LAO/STO superlattices prepared by pulsed laser deposition unambiguously reveal Ti 3d carriers with localized and itinerant character. These dual character carriers are generated during the building of the LAO/STO interfaces. Systematic studies on samples before and after annealing under O2 atmosphere and high temperature show that the dual character carriers can be either induced by electron transfer due to the polar-discontinuity or by oxygen vacancies defects. Electrons transferred and donated by oxygen vacancies are microscopically equal in balancing the built-up electric potential. |
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