Electron accumulation in LaAlO₃/SrTiO₃ interfaces by the broken symmetry of crystal field

Using ab initio calculations, we studied structural characteristics of LaAlO₃/SrTiO₃ interface and related with its extraordinarily increased electric conductivity. In both of the two (LaAlO ₃)_m/SrTiO₃ heterojunction models (m = 3;5), the oxygen atoms in the cells were displaced toward the ntype interface and the Ti-centered octahedron structure was compressed along the [001] direction by the atomic reconstructions at the (LaAlO₃)_m/(SrTiO ₃)₄ interfaces. As a result, the 3d_xy orbital of the Ti atom was preferentially occupied due to the lowered energy state of the 3d_xy orbital, which arises from the crystal field asymmetry. We reason that the extra electrons occupy the 3d_xy orbital are accumulated at the interface by the displacement of the oxygen atoms. This accumulation contributes to the conductivity of the n-type interface.