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Two-dimensional electron liquid and a key missing piece of the “polar catastrophe” puzzle


The strongly correlated electrons that form a two-dimensional electron liquid (2-DEL) at the interface between the insulators, LaAlO3 and SrTiO3, has fueled interesting research on emergent collective phenomena, including superconductivity and magnetism. These collective phenomena distinguish this rich system from conventional two-dimensional electron gases at compound semiconductor interfaces. Averting a polar catastrophe – a diverging potential due to the polar discontinuity at the LaAlO3 and TiO2-terminated (100) SrTiO3 interface – proposed as the origin of this 2-DEL, however, has been highly debated with focus on the role of defects in the SrTiO3 while the LaAlO3 has been assumed perfect. In this talk I will discuss our experiments and first principles calculations, which show that the cation stoichiometry of the nominal LaAlO3 layer is key to 2-DEL formation1: 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. While the requirement of a critical thickness has been known2, our experiments find a missing piece of the polar catastrophe puzzle – a critical La/Al ratio (determined to be ! 0.97±0.03), which is also required for the formation of a 2-DEL.

1. M. P. Warusawithana et. al., Nature Communications (accepted 2013).
2. S. Thiel et. al., Science 313, 1942-1945(2006).