Charge transfer in iridate-nickelate superlattices*

The construction of superlattices, unit-cell-scale layering of two or more materials at the nanoscale, allows one to access novel properties not possessed by the constituent materials. Here, we show that in superlattices consisting of strontium iridate and lanthanum nickelate, up to one electron can transfer from iridium to nickel at the interfaces, leading to Ni²(d⁸) and the unusual oxidation state Ir⁵⁺(d⁴). Using density functional theory based calculations including a Hubbard U correction and spin-orbit coupling, we characterize the 1:1 superlattices with both (001) and (111) ordering, allowing us to explore the competition among Hund's coupling, the crystal field splitting and spin-orbit coupling. We find that the spin-orbit coupling present for iridium does not completely dominate the d⁴ configuration, which would result in a nonmagnetic state with a filled J_eff=3/2 manifold, but competes with the Hund's coupling and crystal field splitting. In these superlattices, the surprising competition in the energy landscape results in Ir⁵⁺ with a non-zero magnetic moment.