S. Baronio, C. C. Au-Yeung, B. A. Davidson, R. J. Green, C. S. B. Pang, R. Sutarto, A. Damascelli, K. Zou, G. A. Sawatzky

High-temperature superconductivity has been reported in La3Ni2O7 under high pressure in bulk crystals, and under compressive epitaxial strain in thin films. This compound often exhibits natural polymorphism in bulk crystals, consisting of mixed bilayer (2222) and an alternating monolayer-trilayer (1313) phases. The electronic structure of these phases and their respective roles in superconductivity remain unclear.

We synthesized phase-pure 2222 and 1313 thin films by molecular beam epitaxy (MBE), to investigate their layer-dependent element distribution and electronic configuration using resonant x-ray reflectometry (RXR) [1]. We applied a universal method recently developed by our group [2] to grow high-quality nickelates thin films, characterized by x-ray diffraction. RXR provides depth-resolved chemical profiling with sensitivity to each element’s valence, enabling determination of the Ni 3d orbital occupation and oxygen hole distribution within the unit cell; in addition, polarization-dependent RXR reveals the orbital character of the oxygen holes across different layers. This approach makes it possible to study how the layering sequence affects the individual electronic structure of the two polymorphs, and consequently their macroscopic physical properties such as spin/charge density waves and superconductivity.

[1] S. Macke et al., Adv. Mater. 26, 6554 (2014)

[2] B. A. Davidson et al., Nat. Commun. 16, 8587 (2025)