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

Abstract

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${}_{0.35}$Sr${}_{0.65}$)MnO${}_{3}$/SrTiO${}_{3}$, with a primitive polar discontinuity of (Nd${}_{0.35}$Sr${}_{0.65}$O)${}^{0.35+}$-(TiO${}_{2}$)${}^{0}$, were thoroughly investigated. (Nd${}_{0.35}$Sr${}_{0.65}$)MnO${}_{3}$ 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${}_{0.35}$Sr${}_{0.65}$)MnO${}_{3}$ was firmly resolved with a length scale of \ensuremath{\sim}2 nm and a charge density on the order of \ensuremath{\sim}10${}^{13}$/cm${}^{2}$ and is effectively mediated by an asymmetric Ti interdiffusion. The intricate electronic correlations of the interfacial (Nd${}_{0.35}$Sr${}_{0.65}$)MnO${}_{3}$ 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.