Two-dimensional metallic behavior at polar MgO/BaTiO₃ (110) interfaces

The first-principles calculations are employed to investigate the electrical properties of polar MgO/BaTiO₃ (110) interfaces. Both n-type and p-type polar interfaces show a two-dimensional metallic behavior. For the n-type polar interface, the interface Ti 3d electrons are the origin of the metallic and magnetic properties. Varying the thickness of BaTiO₃ may induce an insulator-metal transition, and the critical thickness is 4 unit cells. For the p-type polar interface, holes preferentially occupy the interface O 2p_y state, resulting in a conducting interface. The unbalance of the spin splitting of the O 2p states in the interface MgO layer leads to a magnetic moment of about 0.25μ_B per O atom at the interface. These results further demonstrate that other polar interfaces, besides LaAlO₃/SrTiO₃, can show a two-dimensional metallic behavior. It is helpful to fully understand the role of polar discontinuity on the properties of the interface, which widens the field of polar-nonpolar interfaces.