Tunable conductivity threshold at polar oxide interfaces

The physical mechanisms responsible for the formation of a two-dimensional electron gas at the interface between insulating SrTiO₃ and LaAlO₃ have remained a contentious subject since its discovery in 2004. Opinion is divided between an intrinsic mechanism involving the build-up of an internal electric potential due to the polar discontinuity at the interface between SrTiO₃ and LaAlO₃, and extrinsic mechanisms attributed to structural imperfections. Here we show that interface conductivity is also exhibited when the LaAlO₃ layer is diluted with SrTiO₃, and that the threshold thickness required to show conductivity scales inversely with the fraction of LaAlO₃ in this solid solution, and thereby also with the layer's formal polarization. These results can be best described in terms of the intrinsic polar-catastrophe model, hence providing the most compelling evidence, to date, in favour of this mechanism.