Aspects of the synthesis of thin film superconducting infinite-layer nickelates

The recent observation of superconductivity in Nd_0.8Sr_0.2NiO₂ calls for further investigation and optimization of the synthesis of this 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 SrTiO₃ (001) substrates and the subsequent topotactic reduction. We find that to achieve single-crystalline, single-phase superconducting Nd_0.8Sr_0.2NiO₂, it is essential that the precursor perovskite Nd_0.8Sr_0.2NiO₃ thin film is stabilized with no visible 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 SrTiO₃ 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_0.8Sr_0.2NiO₂ (001) epitaxial thin films up to ∼10 nm.