Superconductivity in Ruddlesden–Popper nickelates: a review of recent progress, focusing on thin films

The discovery of superconductivity withK in the nickelate Ruddlesden-Popper (RP) bilayer LaNiOat high pressure has opened a new platform for unconventional superconductivity, followed by the subsequent observation of superconductivity in trilayer LaNiO, also at high pressure. Remarkably, ambient-pressure superconductivity was also observed recently in LaNiOultra-thin films when grown on substrates that provide compressive strain. This discovery significantly extends the type of experimental techniques that can be used in nickelates, previously limited due to the high-pressure constraint. Discussing the similarities and differences among these nickel oxides will provide new insights into understanding the mechanism of high-superconductivity in correlated electron systems. In this paper, we review the experimental and theoretical progress on RP nickelates, with emphasis on thin films, and discuss future perspectives and research directions.