  Emergent phenomena, including superconductivity and magnetism, found in the
two-dimensional electron liquid (2-DEL) at the interface between the insulators
LaAlO3 and SrTiO3 distinguish this rich system from conventional
two-dimensional 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 SrTiO3 while the LaAlO3 has been assumed perfect. Our experiments and first
principles calculations show that the cation stoichiometry of the nominal
LaAlO3 layer is key to 2-DEL formation: only Al-rich LaAlO3 results in a 2-DEL.
While extrinsic defects including oxygen deficiency are known to render
LaAlO3/SrTiO3 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 a roadmap for identifying other interfaces
where emergent behaviors await discovery.
