Aspects of the Synthesis of Thin Film Superconducting Infinite-Layer Nickelates

Abstract: The recent observation of superconductivity in Nd${0.8}$Sr${0.2}$NiO${2}$ calls for further investigation and optimization of the synthesis of this metastable infinite-layer nickelate structure. Here, we present our current understanding of important aspects of the growth of the parent perovskite compound via pulsed laser deposition on SrTiO${3}$ (001) substrates, and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd${0.8}$Sr${0.2}$NiO${2}$, it is essential that the precursor perovskite Nd${0.8}$Sr${0.2}$NiO${3}$ thin film is stabilized with high crystallinity and no impurity phases; in particular, a Ruddlesden-Popper-type secondary phase is often observed. We have further investigated the evolution of the soft-chemistry topotactic reduction conditions to realize full transformation to the infinite-layer structure with no film decomposition or formation of other phases. We find that capping the nickelate film with a subsequent SrTiO${3}$ layer provides an epitaxial template to the top region of the nickelate film, much like the substrate. Thus, for currently optimized growth conditions, we can stabilize superconducting single-phase Nd${0.8}$Sr${0.2}$NiO${2}$ (001) epitaxial thin films up to ~ 10 nm.