Enhancing superconductivity in the Ruddlesden-Popper bilayer nickelates via tuning Ni crystal-field-splitting

Since the discovery of superconductivity in both bulk and thin-film Ruddlesden–Popper bilayer nickelates, the superconducting transition temperature T_c has continued to rise through chemical alloying in La_3-xR_xNi₂O₇ (where R is a non-La rare-earth element). In this work, we propose a new strategy to further enhance T_c in La_3-xR_xNi₂O₇ by engineering the spatial distribution of -element substitution. Combining first-principles calculations with superconducting gap-equation analysis, we investigate the prototypical compound La₃SmNi₂O₇, which currently exhibits the highest T_c among this family. We find that Sm atoms energetically prefer to occupy the outer layers of the 327 block. More importantly, we discover that superconductivity is enhanced when all Sm atoms reside on the same outer layer (“one-sided” configuration), compared with a symmetric distribution across both outer layers (“two-sided” configuration). The “one-sided” configuration is experimentally achievable via thin-film growth techniques. We attribute the enhanced T_c to the increased Ni crystal-field splitting induced by the Sm–La asymmetric chemical environment, which amplifies spin fluctuations. Our findings establish a positive correlation between T_c and Ni crystal-field splitting in Ruddlesden–Popper bilayer nickelates and suggest a practical route to further raise T_c in experiment.