  Understanding the nature of electrical conductivity, superconductivity and
magnetism between layers of oxides is of immense importance for the design of
electronic devices employing oxide heterostructures. We demonstrate that
resonant inelastic X-ray scattering can be applied to directly probe the
carriers in oxide heterostructures. Our investigation on epitaxially grown
LaAlO3/SrTiO3 superlattices unambiguously reveals the presence of both
localized and delocalized Ti 3d carriers. These two types of carriers are
caused by oxygen vacancies and electron transfer due to the polar discontinuity
at the interface. This result allows explaining the reported discrepancy
between theoretically calculated and experimentally measured carrier density
values in LaAlO3/SrTiO3 heterostructures.
