The physical mechanisms responsible for the formation of a two-dimensional electron gas atthe interface between insulating srTiO_3 and LaAlO_3 have remained a contentious subject sinceits discovery in 2004. opinion is divided between an intrinsic mechanism involving the build-upof an internal electric potential due to the polar discontinuity at the interface between srTiO_3and LaAlO_3, and extrinsic mechanisms attributed to structural imperfections. Here we showthat interface conductivity is also exhibited when the LaAlO_3 layer is diluted with srTiO_3, andthat the threshold thickness required to show conductivity scales inversely with the fraction ofLaAlO_3 in this solid solution, and thereby also with the layer’s formal polarization. These resultscan be best described in terms of the intrinsic polar-catastrophe model, hence providing themost compelling evidence, to date, in favour of this mechanism.
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