Coexistence of surface oxygen vacancy and interface conducting states in LaAlO₃/SrTiO₃ revealed by grazing-angle resonant soft x-ray scattering

Oxide heterostructures have shown rich physics phenomena, particularly in the conjunction of exotic insulator-metal transition (IMT) at the interface between polar insulator LaAlO3 and non-polar insulator SrTiO3 (LaAlO3/SrTiO3). The polarization catastrophe model has suggested an electronic reconstruction, yielding to metallicity at both the interface and surface. Another scenario is the occurrence of surface oxygen vacancy at LaAlO3 (surface-O-v), which has predicted surface-to-interface charge transfer, yielding metallic interface but insulating surface. To clarify the origin of IMT, one should probe surface-O-v and the associated electronic structures at both the surface and the buried interface simultaneously. Here, using grazing-angle resonant soft x-ray scattering (GA-RSXS) supported with first-principles calculations, we reveal the co-existence of the surface-O-v state and the interface conducting state only in conducting LaAlO3/SrTiO3 (001) films. Interestingly, both the surface-O-v state and the interface conducting state are absent for the insulating film. As a function of O-v density, while the surface-O-v state is responsible for the IMT, the spatial charge distribution is found responsible for a transition from two-dimensional-like to three-dimensional-like conductivity accompanied by spectral weight transfer, revealing the importance of electronic correlation. Our results show the importance of surface-O-v in determining interface properties and provide a new strategy in utilizing GA-RSXS to directly probe the surface and buried interface electronic properties in complex oxide heterostructures.