Experimental Progress in Ambient-Pressure Superconducting Bilayer Nickelate Films

TL;DR

This review summarizes recent experimental advances in ambient-pressure superconducting bilayer nickelate films, focusing on synthesis, properties, and unresolved issues in understanding their high-temperature superconductivity.

Contribution

It provides a comprehensive overview of synthesis methods, electronic structure, and key challenges in ambient-pressure bilayer nickelate superconductors.

Findings

Superconductivity observed near 80 K at ambient pressure in bilayer nickelate films.

Epitaxial strain can mimic high-pressure structural features.

Several issues remain, including reproducibility and pairing mechanisms.

Abstract

Bilayer Ruddlesden-Popper nickelates display superconductivity near 80 K under high pressure, establishing a new nickelate platform for studying unconventional high-temperature superconductivity. The recent stabilization of superconducting RA3Ni2O7 (RA = rare earth or alkaline earth) films at ambient pressure has changed the experimental landscape: epitaxial strain can reproduce key structural ingredients of the high-pressure phase while making transport, spectroscopy, microscopy, and device-oriented measurements directly accessible. This Review summarizes the experimental progress on ambient-pressure superconducting bilayer nickelate films, with emphasis on synthesis routes, oxygen stoichiometry, substrate-induced strain, normal-state transport, superconducting properties, doping phase diagrams, and momentum-resolved electronic structure. We highlight several issues that remain unsettled, including the reproducibility of phase-pure ultrathin films, the microscopic origin of the two-step superconducting transition, the role of oxygen defects and substrate-derived doping, the position of the Ni 3dz2-derived {\gamma} band, and the pairing symmetry. We close by outlining experimental directions that could establish a more quantitative link among crystal structure, orbital reconstruction, and superconductivity in bilayer nickelate films.