Multiorbital structure of the two-dimensional electron gas in LaAlO3/SrTiO3 heterostructures: The formation of a dx y ferromagnetic sheet

We demonstrate the formation of a ferromagnetic two-dimensional dxy electron sheet strictly confined to the TiO2 interface layer in LaAlO3/SrTiO3 heterostructures. Based on first-principles density functional calculations we show that the complex subband structure of the two-dimensional electron gas (2DEG) generated at the LaO/TiO2 (001) interface is universal, and almost independent of the SrTiO3 thickness. It is composed of a ladder of dxy states of light electrons and only one degenerate dxz,yz heavier subband. All the states are spin polarized although the exchange splitting is only significant for the lowest energy dxy subband, which leads to magnetic moments ferromagnetically coupled and localized at the interface. The SrTiO3 ferroelectric-like lattice distortions determine the subband occupation and therefore their orbital character, exchange splitting, and charge density profile. The complex structure of the 2DEG can explain the coexistence in the same sample of superconductivity and magnetism.