  Conventional two-dimensional electron gases are realized by engineering the
interfaces between semiconducting compounds. In 2004, Ohtomo and Hwang
discovered that an electron gas can be also realized at the interface between
large gap insulators made of transition metal oxides [1]. This finding has
generated considerable efforts to clarify the underlying microscopic mechanism.
Of particular interest is the LaAlO3/SrTiO3 system, because it features
especially striking properties. High carrier mobility [1], electric field
tuneable superconductivity [2] and magnetic effects [3], have been found. Here
we show that an orbital reconstruction is underlying the generation of the
electron gas at the LaAlO3/SrTiO3 n-type interface. Our results are based on
extensive investigations of the electronic properties and of the orbital
structure of the interface using X-ray Absorption Spectroscopy. In particular
we find that the degeneracy of the Ti 3d states is fully removed, and that the
Ti 3dxy levels become the first available states for conducting electrons.
