Structural Basis for Superconductivity in Strain-Engineered Bilayer Nickelate Thin Films

The origin of superconductivity in Ruddlesden-Popper nickelates remains a key question, particularly regarding the importance of certain band alignments and related hopping parameters which are sensitive to structural distortions. Recently, compressive epitaxial strain has been shown to stabilize superconductivity in bilayer La₃Ni₂O₇ [1], potentially mimicking that observed in bulk single crystals under high hydrostatic pressure [2]. In bulk samples, previous proposals highlighted c-axis compression as a key driver of low-energy Ni 3d bands near the Fermi level. Intriguingly, this lattice parameter instead expands in compressively strained superconducting films. Other proposals have suggested sensitive dependence of the superconducting pairing symmetry on subtle changes in the nickel-oxygen bonding environment, calling for precise measurements of the local atomic structure in these compounds. Multislice electron ptychography (MEP) provides a method to precisely extract reliable oxygen atomic positions with deep sub-Ångström spatial resolution [3]. Here, we leverage MEP to quantitatively investigate a full series of epitaxial thin films spanning compressive to tensile strain. We track the strain-dependent evolution of key structural parameters such as Ni-O bond lengths, bond angles, and octahedra. This structural parameterization provides crucial input for accurate theoretical modeling in these compounds which is not accessible by other measurements. We further introduce a density functional theory (DFT) framework for strain decomposition to identify key commonalities in the lattice and electronic structures of superconducting sample geometries in both bulk and thin films. [4]



1. Ko et al. Nature 638, 935 (2025).

2. Sun et al. Nature 621, 493 (2023).

3. Chen et al. Science 372, 826 (2021).

4. Bhatt et al. arXiv:2501.08204 (2025).