An interface between the insulating oxides LaAlO3 and SrTiO3 can be metallic with an extremely high carrier mobility, the valence mismatch at the interface leading to the transfer of half an electron per unit cell from LaAlO3 (band gap 5.6 eV) to SrTiO3 (band gap 3.2 eV), from the LaO interface layer to the TiO2 layer. Samples prepared under increasing oxygen pressure exhibit a large increase of the sheet resistance which decreases on increasing the temperature from 10K to 70K as well as a large negative magnetoresistance and magnetic hysteresis at low temperatures. These properties are presumably characteristic of intrinsic interfaces.
To gain some insight into the origin of the observed magnetoresistance, we used parameter-free electronic structure calculations to show that the half electron (per interface Ti ion) transferred from the LaO layer on one side of the interface to the TiO2 layer on the other side favours a rotation of TiO6 octahedra just as it does in bulk LaTiO3 (see Figure). As in bulk LaTiO3, the distortion is crucial to the formation of a charge-ordered antiferromagnetic (AFM) insulating ground state and other charge, magnetic and orbital properties, and results in a characteristic buckling of the Ti-O-Ti bonding at the interface. Ionic relaxation plays a crucial role in determining the localization of the extra electron on the interface Ti ions and leakage into the bulk layer. However, charge ordering suppresses this leakage even when relaxation is included and, in the charge-ordered states the electrons are strongly localized at the interface. Our calculations suggest an explanation for recent experimental results in terms of the proximity of an AFM insulating ground state to a metallic ferromagnetic excited state (or an insulating ferromagnetic state with reduced band gap).
Z. Zhong and P.J. Kelly, Electronic-structure–induced reconstruction and magnetic ordering at the LaAlO3|SrTiO3 interface, Europhysics Letters 84, 27001 (2008).
Figure. Relaxed LaAlO3|SrTiO3 interface structure and charge density iso-surface of the surplus electron for the charge-ordered ferromagnetic state. The orthorhombic translation vectors are a, b, c.