Parallel electron-hole bilayer conductivity from electronic interface reconstruction: Theory

The polar discontinuity in oxide heterostructures and thin films can lead to novel electronic states even if simple band insulators such as LaAlO$_3$ and SrTiO$_3$ are involved. Density functional theory calculations [1] show that a strong lattice polarization allows several layers of LaAlO$_3$ to remain insulating before an electronic reconstruction takes place at around 4 monolayers (MLs) [2]. Here we demonstrate that a capping SrTiO$_3$ layer can trigger the insulator-to-metal transition already at two MLs of LaAlO$_3$. A surface O 2$p$ state is identified as the origin of this additional band shift. Altogether, a SrTiO$_3$-capping layer represents an alternative pathway to tune the electronic reconstruction of the system and to realize an electron-hole bilayer in the search, e.g., for novel excitonic phases. \newline [1] R. Pentcheva and W.E. Pickett, Phys. Rev. Lett. {\bf 102}, 107602 (2009). \newline [2] S.Thiel et al., Science {\bf 313}, 1942 (2006).