  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).
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-Ov), which has
predicted surface-to-interface charge transfer yielding metallic interface but
insulating surface. To clarify the origin of IMT, one should probe surface-Ov
and the associated electronic structures at both the surface and the buried
interface simultaneously. Here, using low-angle resonant soft X-ray scattering
(LA-RSXS) supported with first-principles calculations, we reveal the
co-existence of the surface-Ov state and the interface conducting state only in
conducting LaAlO3/SrTiO3 (001) films. Interestingly, both the surface-Ov state
and the interface conducting state are absent for the insulating film. As a
function of Ov density, while the surface-Ov state is responsible for the IMT,
the spatial charge distribution is found responsible for a transition from
two-dimensional-like to three-dimensional-like conducting accompanied by
spectral weight transfer, revealing the importance of electronic correlation.
Our results show the importance of surface-Ov in determining interface
properties and provides a new strategy in utilizing LA-RSXS to directly probe
the surface and buried interface electronic properties in complex oxide
heterostructures.
