Orbital Reconstruction and the Two-Dimensional Electron Gas at theLaAlO3/SrTiO3Interface

In 2004, Ohtomo and Hwang discovered that an electron gas is created at the interface between insulating ${\mathrm{LaAlO}}_3$ and ${\mathrm{SrTiO}}_3$ compounds. Here we show that the generation of a conducting electron gas is related to an orbital reconstruction occurring at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ 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 $3d_{xy}$ levels become the first available states for conducting electrons.