  One of the boundary conditions of the classical electromagnetic theory
demands continuous electric potential across any boundary, which may not be
naturally satisfied in atomically engineered heterostructures. Such polarity
mismatch in oxide heterointerfaces is compensated through some
(electronic/chemical/structural) reconstructions, leading to a myriad of
emergent phenomena. The question we are posing is whether conventional
semiconductor band bending framework is sufficient to comprehend compensation
mechanisms in oxide heterostructures since, unlike semiconductors, complex
oxides host strong electron correlations whose effects are indispensable. To
address this, we investigate the interface between a prototypical insulating
double perovskite Nd$_2$NiMnO$_6$ and a wide-bandgap insulator SrTiO$_3$. This
polar/non-polar interface offers a similar scenario as the famous
LaAlO$_3$/SrTiO$_3$ system but with an exception - two transition metal sites
with two individual correlated energy scales. By combining several experimental
techniques and density functional theory, we establish a site-selective charge
compensation process that occurs explicitly at the Mn site of the film, leaving
the Ni sites inert. This surprising selectivity, which cannot be accounted by
existing polar compensation mechanisms, is directly attributed to the TM
cations' relative correlation energy scales. This discovery presents that
site-specific charge compensation can be a designers tool for tailoring
emergent phenomena in oxide heterostructures.
