Comparison of Electronic and Spin Excitations in Different Polymorphs of the Nickelate Superconductor La3Ni2O7

High-temperature superconductivity in La₃Ni₂O₇ has garnered significant attention over the past year. Although there is substantial experimental evidence indicating the presence of superconductivity above 80 K under under hydrostatic pressure, conclusive evidence identifying the precise superconducting structure remains elusive. This is large part due to the identification of two different structural polymorphs : the expected Ruddlesden-Popper bilayer structure (dubbed 2222), and a naturally occurring superlattice of single and trilayer blocks of NiO₆ octahedra (dubbed 1313), with superconductivity being reported in both polymorphs. Experimentally distinguishing the properties of each of these polytypes is critical. Due to the unavoidable presence of impurity phases and stacking faults in bulk crystals, we have employed layer-by-layer growth of thin films of both the 2222 and 1313 polymorphs using oxide molecular beam epitaxy. We investigated both polytypes using a combination of x-ray absorption spectroscopy, resonant elastic and inelastic soft x-ray scattering to compare the different electronic structure, spin excitations, and presence of spin density wave order in these two polytypes. Our results provide clear evidence of differences between the 2222 and 1313 phases, shedding light on the possible origins of superconductivity in La₃Ni₂O₇.