  The discovery of conducting two-dimensional electron gas (2DEG) and magnetism
at the interface between insulating nonmagnetic oxides, as exemplified by the
polar LaAlO3 and nonpolar SrTiO3 has raised prospects for attaining interfacial
functionalities absent in the component materials. Yet, the microscopic origin
of such emergent phenomena remains unclear, posing obstacles to design of
improved functionalities. Using first principles defect calculations, we reveal
a unifying polarity-induced defect mechanism for both conductivity and
magnetism at polar-nonpolar interfaces of nonmagnetic insulating oxides. We
demonstrate that the polar-discontinuity across the interface triggers
thermodynamically the spontaneous formation of certain defects that in turn
cancel the polar field induced by the polar discontinuity. It turns out that
the 2DEG originates from those spontaneously formed surface donor defects
(oxygen vacancy), but the density of 2DEG is controlled by the interfacial
anti-site acceptor defects (Al-on-Ti). The interface magnetism is found to
originate from the un-ionized deep Ti-on-Al anti-site donor defects within the
LaAlO3 side near the interface. Our results suggest practical design guidelines
for inducing and controlling both 2DEG and magnetism at polar-nonpolar oxide
interfaces.
