Orbital reconstruction at the LAO/STO interface investigated by x-ray spectroscopy

The appearance of high mobility electrons at the LaAlO(3)/SrTiO(3) (LAO/STO) interface has raised strong interest in the material science community and a lively debate on the origin of the phenomenon. In particular, in view of the large band gaps of the two bulk single crystals constituting this heterostructure, the realization of a conducting system was totally unexpected. A possible explanation is an electronic reconstruction of the interface, realizing a transfer of electrons from the LaAlO(3) surface to SrTiO(3) near the interface, thereby avoiding the polarization catastrophe associated with the alternating polar layers of the LaAlO(3) film. The predictions of theoretical models based on this idea are quite peculiar and need to be verified by specific experiments able to address the electronic properties of the LAO/STO buried interface. Here, by using x-ray spectroscopy techniques, we show that the appearance of an electron system is correlated to the removal of the degeneracy of the titanium 3d states, and doped electrons appear in a band preferentially created by the hybridization between 3d(xy) states of titanium and oxygen 2p(x,y) states. This splitting is consistent with an ordering of the Ti 3d(xy) orbital belonging to the TiO(6) octahedra close to the interface, as theoretically proposed. However, the valence of titanium ions remains prevalently 4+, therefore other mechanisms should be also considered for the stabilization of the system.