Synthesis of layered square-planar Lanthanum nickelate thin films, La_n+1Ni_nO_2n+2

The layered square-planar nickelates, R_n+1Ni_nO_2n+2 (R = trivalent rare-earth cation and n > 1), are an exciting platform to tune the properties of superconducting square-planar nickelates via dimensionality [1-2]. To date, superconducting Nd_n+1Ni_nO_2n+2 films have been achieved only on NdGaO₃ due to the competing requirements for the synthesis of the precursor Ruddlesden-Popper, Nd_n+1Ni_nO_3n+1, and reduction to the square-planar phase [3]. On SrTiO₃, the high tensile strain in the precursor phase, Nd_n+1Ni_nO_3n+1, leads to a high density of stacking faults [3]. Improved crystalline quality and an enhanced superconducting transition temperature have been achieved in (Nd,Sr)NiO₂ on LSAT by lowering the tensile strain in the parent perovskite phase [4]. Here, we investigate the role of epitaxial strain in the synthesis of Lanthanum-based layered nickelates on NdGaO₃ and SrTiO₃. We demonstrate a substantially higher crystalline quality in La_n+1Ni_nO_3n+1 on NdGaO₃ than Nd_n+1Ni_nO_3n+1, due to the lower tensile strain. Furthermore, we synthesize high crystalline quality La_n+1Ni_nO_3n+1 on SrTiO₃, unattainable in Nd_n+1Ni_nO_3n+1 due to the excessive lattice-mismatch. Finally, we discuss the reduction to the square-planar phase on NdGaO₃ and SrTiO₃.

[1] Li, D. et al. Superconductivity in an infinite-layer nickelate. Nature 572, 624–627 (2019).

[2] Pan, G. A. et al. Superconductivity in a quintuple-layer square-planar nickelate. Nat Mater 1–5 (2021).

[3] Ferenc Segedin, D. et al. Limits to the strain engineering of layered square-planar nickelate thin films. Nat. Commun. 14, 1468 (2023).

[4] Lee, K. et al. Linear-in-temperature resistivity for optimally superconducting (Nd,Sr)NiO2. Nature 619, 288–292 (2023).