On the Origin of the Two-Dimensional Electron Gas between LaAlO3 and SrTiO3
A thin film (a few nm) of LaAlO3 (LAO) on a SrTiO3 (STO) substrate gives rise to a two-dimensional electron gas at the interface of interesting and surprising properties. This is in spite of both materials being quite unassuming band insulator perovskites, in spite of the growth being epitaxial, and in spite of both materials being centrosymmetric, thus, with same bulk polarisation a priori.
The origin of the gas is, nevertheless, a polar discontinuity at the interface, stemming from the fact that both insulators are of different kinds: STO has zero bulk polarisation, while LAO has half a quantum, the two allowed values for centrosymmetric materials, corresponding to Berry phases zero and pi. After reviewing this, consequences will be explored for the possible instabilities related to the polar discontinuity (electronic reconstruction, the stabilisation of redox defects on the surface), as well as for similar instabilities in different systems towards one-dimensional electron gases (in stepped LAO/STO systems and in honeycomb planar insulators).