A spin-orbit coupling-induced two-dimensional electron gas in BiAlO₃/SrTiO₃ heterostructures

Both LaAlO₃ and BiAlO₃ are isostructural, isoelectronic and band insulators. Therefore, in analogy to the LaAlO₃/SrTiO₃ heterostructure, a quasi two dimensional electron gas (q-2DEG) could be anticipated in BiAlO₃/SrTiO₃ heterostructures. Our density functional theory based scalar relativistic calculations show that BiAlO₃/SrTiO₃ heterostructures remain insulating for a BiAlO₃ film thickness up to 5 unit cells. However, with spin orbit coupling included in the crystal Hamiltonian, we find a thickness dependent insulator to metal transition for BiAlO₃/SrTiO₃ heterostructures. However, unlike the Ti³⁺/Ti⁴⁺ electronic reconstruction in LaAlO₃/SrTiO₃, the conductivity in BiAlO₃/SrTiO₃ is found to originate from the subsurface Bi 6p states. The results suggest that the properties of q-2DEG in BiAlO₃/SrTiO₃ can be controlled using an external electric field, leading to a wide range of solid state applications.