The polar discontinuity at the SrTiO${}_3$/LaAlO${}_3$ interface (STO/LAO) can in principle sustain an electron density of $3.3\times {10}^{14}$ cm${}^{-2}$ (0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schrödinger-Poisson simulations we show that the problem lies in the asymmetric nature of the structure, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, or to fully passivate the LAO surface. Our insights apply to oxide interfaces in general, explaining for instance why the SrTiO${}_3$/GdTiO${}_3$ interface has been found to exhibit the full density of $3.3\times {10}^{14}$ cm${}^{-2}$.

• Received 18 October 2012

DOI:https://doi.org/10.1103/PhysRevB.86.241108