The polar discontinuity picture in nickelate superlattices: comparison of the insulating and metallic phases

Abstract

We present a first-principles study of the formal-charge polar discontinuity in superlattices of BaTiO$_3$ interfaced with SmNiO$_3$ or BiNiO$_3$, comparing the results for the insulating and metallic nickelate phases. Polar discontinuities arising from differently charged layers in perovskites are well-studied in heterostructures such as LaAlO$_3$/SrTiO$_3$, and recently, signatures consistent with such a polar discontinuity have been observed in systems where the formally charged layers would reside in a metal. We first identify signatures of the polar discontinuity in the structure and the density of states of superlattices with BaTiO$_3$ and insulating BiNiO$_3$, and find partial screening by modification of the BiNiO$_3$ charge disproportionation pattern analogous to our previous study on BaTiO$_3$/SmNiO$_3$. We then examine superlattices constructed from the corresponding metallic nickelate phases and find that, despite the presence of screening charges, the qualitative interfacial response is remarkably similar to that of the insulating counterparts. We show that the metallic screening charges have little influence on the apparent formal-charge polar discontinuity and discuss practical limitations of the polar-discontinuity picture when interpreting and quantifying screening at metallic nickelate/ferroelectric interfaces.