Emergent phenomena, including superconductivity and magnetism, found in the two-dimensional electron liquid (2-DEL) at the interface between the insulators lanthanum aluminate (LaAlO 3 ) and strontium titanate (SrTiO 3 ) distinguish this rich system from conventional 2D electron gases at compound semiconductor interfaces. The origin of this 2-DEL, however, is highly debated, with focus on the role of defects in the SrTiO 3 , while the LaAlO 3 has been assumed perfect. Here we demonstrate, through experiments and firstprinciple calculations, that the cation stoichiometry of the nominal LaAlO 3 layer is key to 2-DEL formation: only Al-rich LaAlO 3 results in a 2-DEL. Although extrinsic defects, including oxygen deficiency, are known to render LaAlO 3 /SrTiO 3 samples conducting, our results show that in the absence of such extrinsic defects an interface 2-DEL can form. Its origin is consistent with an intrinsic electronic reconstruction occurring to counteract a polarization catastrophe. This work provides insight for identifying other interfaces where emergent behaviours await discovery.

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During the preparation of the manuscript, panels D and E in Figure 5 were inadvertently mislabeled. The correctly labeled panels and their corresponding legends appear below.(D) miR17-5p and let7b had no effect on the proliferation of SVEC. Thirty-six hours after transfection of the indicated oligos into SVEC, 5 × 10 4 cells were seeded, and the number of cells was determined by cell counting at the indicated time points. Data are presented as mean ± SD (n = 4 per group, P = NS). (E) miR17-5p and let7b had no effect on the motility of SVEC. In vitro wound healing was quantified as the average length of the elongation of wound edges at 4 specific points over 24 hours. Data are presented as mean ± SD (n = 3 per group, P = NS).The JCI regrets the error.