Atomic-scale observation of a graded polar discontinuity and a localized two-dimensional electron density at an insulating oxide interface

Using atomically resolved electron energy-loss spectroscopy, the atomic-plane-by-atomic-plane, unit-cell-by-unit-cell stoichiometry, and charge characteristics of the oxide interface (Nd_0.35Sr_0.65)MnO₃/SrTiO₃, with a primitive polar discontinuity of (Nd_0.35Sr_0.65O)^0.35+-(TiO₂)⁰, were thoroughly investigated. (Nd_0.35Sr_0.65)MnO₃ is a strongly correlated insulator and the interface was characterized to be insulating. The cell-specific stoichiometric evaluation unveiled an extensive interdiffusion across the interface. The plane-specific charge characterization revealed that the interdiffusion grades the primitive polar discontinuity. Despite the graded polar discontinuity, a charge transfer inversely into (Nd_0.35Sr_0.65)MnO₃ was firmly resolved with a length scale of ∼2 nm and a charge density on the order of ∼10¹³/cm² and is effectively mediated by an asymmetric Ti interdiffusion. The intricate electronic correlations of the interfacial (Nd_0.35Sr_0.65)MnO₃ unit cells and the interdiffusion-induced chemical disorder tend to render the charges localized, resulting in a localized two-dimensional electron density and thus the insulating interface, in distinct contrast to the conventional understanding of a vanishing charge density for an insulating interface and the metallic two-dimensional electron gas found at other classical polar-discontinuous interface systems. A potential strain manipulation on the electronic localization of the electron density was also proposed.