Strain engineering of layered square-planar nickelate thin films*

The discovery of superconductivity in infinite-layer Nd_0.8Sr_0.2NiO₂ thin films reignited an interest in the nickelates as cuprate analogues. More broadly, the infinite-layer nickelates are the n = ∞ member of a homologous series of ‘layered square-planar nickelates’, R_n+1Ni_nO_2n+2 or (RNiO₂)_n(RO₂), where R = trivalent rare-earth cation and n > 1.   These compounds host n quasi-two-dimensional NiO₂ planes separated by (RO₂)⁻ spacer layers, making the layered square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality.  Our work investigates the role of epitaxial strain in the competing requirements for the synthesis of the Ruddlesden-Popper compounds, R_n+1Ni_nO_3n+1, and subsequent reduction to the square-planar phase, R_n+1Ni_nO_2n+2. We synthesize our highest quality Ruddlesden-Popper compounds films under compressive strain and observe a high density of extended defects forms under tensile strain. Films reduced on compressive substrates become insulating and form compressive strain-induced c-axis canting defects, while films on the intermediate strain state are metallic and, for optimal layering, superconducting. This work provides a pathway to the synthesis of Nd_n+1Ni_nO_2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.