Ambient-pressure Ruddlesden-Popper bilayer nickelate superconductors: From discovery to prospects

In recent years, significant progress has been made in the superconductivity of nickelates, with global teams discovering various nickelate superconductors under ambient and high pressure conditions. Research teams in China and USA have independently discovered ambient-pressure superconductivity in Ruddlesden-Popper bilayer nickelate thin films through different technical pathways, establishing a novel platform for probing high-temperature superconducting mechanisms. The Chinese teams have synthesized pure-phase bilayer nickelate films with atomically smooth surfaces by using their proprietary Gigantic-Oxidative Atomic-Layer-by-Layer Epitaxy (GOALL-Epitaxy) technique. After <i>in situ</i> strong oxidation processing of surface, surface-sensitive measurements, such as ARPES, can be conducted on these atomically flat films to reveal the electronic structure of the superconducting phase, and further in-depth experimental research on superconducting mechanisms is expected. Through synergistic efforts in lattice engineering, rare-earth/alkaline-earth element substitution, and interface strain engineering, this system has the potential to achieve higher superconducting transition temperatures.