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Oxide heterostructures have shown rich physics phenomena, particularly in theconjunction of exotic insulator-metal transition (IMT) at the interface betweenpolar insulator LaAlO3 and non-polar insulator SrTiO3 (LaAlO3/SrTiO3).Polarization catastrophe model has suggested an electronic reconstructionyielding to metallicity at both the interface and surface. Another scenario isthe occurrence of surface oxygen vacancy at LaAlO3 (surface-Ov), which haspredicted surface-to-interface charge transfer yielding metallic interface butinsulating surface. To clarify the origin of IMT, one should probe surface-Ovand the associated electronic structures at both the surface and the buriedinterface simultaneously. Here, using low-angle resonant soft X-ray scattering(LA-RSXS) supported with first-principles calculations, we reveal theco-existence of the surface-Ov state and the interface conducting state only inconducting LaAlO3/SrTiO3 (001) films. Interestingly, both the surface-Ov stateand the interface conducting state are absent for the insulating film. As afunction of Ov density, while the surface-Ov state is responsible for the IMT,the spatial charge distribution is found responsible for a transition fromtwo-dimensional-like to three-dimensional-like conducting accompanied byspectral weight transfer, revealing the importance of electronic correlation.Our results show the importance of surface-Ov in determining interfaceproperties and provides a new strategy in utilizing LA-RSXS to directly probethe surface and buried interface electronic properties in complex oxideheterostructures.

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