Atomic-layer-by-layer molecular beam epitaxy of Ruddlesden-Popper nickelates

The seminal discovery of high-temperature superconductivity in cuprates spurred extensive exploration of nickelates, given their structural and property similarities. It took three decades for superconductivity to be discovered in the strongly reduced nickelate Nd0.8Sr0.2NiO2, with a critical temperature Tc ≈ 8 K. In 2023, a tenfold higher Tc was detected in La3Ni2O7 under extreme pressure. In 2025, epitaxially strained thin La3Ni2O7 films were reported to superconduct at the ambient pressure, igniting worldwide exploration of various nickelates. One important research direction is to employ advanced synthesis techniques to improve the film phase purity, uniformity and crystallinity. Another is exploring other nickelate compounds. Here, we report the atomic-layer-by-layer molecular beam epitaxy (ALL-MBE) synthesis of a series of strongly oxidized Ruddlesden-Popper nickelates Lan+1NinO3n+1, with n = 1, 2, …,7, and study of their transport properties. Unexpectedly, the dependence of resistivity on n is exponential. We successfully reproduce the data using a parallel-conductance model, with metallic-like transport in one channel and thermally activated hopping in the other. With increased n, the metallic fraction grows, while the localized one shrinks and the activation energy diminishes. We suggest that the local bond-bending lattice distortions, which lessen as n increases, play a major role.