Abstract
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).