Ambient-pressure superconductivity onset above 40 K in (La,Pr)NiO films.

The discovery of Ruddlesden-Popper (RP) bilayer nickelate superconductors under high pressure has opened a new chapter in high-transition-temperature superconductivity. However, the high-pressure conditions and presence of impurity phases have hindered comprehensive investigations into their superconducting properties and potential applications. Here we report ambient-pressure superconductivity onset above the McMillan limit (40 K) in RP bilayer nickelate epitaxial thin films. Three-unit-cell-thick LaPrNiO pure-phase single-crystal films are grown using the gigantic-oxidative atomic layer-by-layer epitaxy on SrLaAlO substrates. Resistivity measurements and magnetic field responses indicate onset transition temperature of 45 K. The transition to zero resistance shows characteristics consistent with a Berezinskii-Kosterlitz-Thouless (BKT) behaviour, with T = 9 K. The Meissner diamagnetic effect is observed at 8 K by using a mutual inductance setup, in agreement with the BKT-like transition. In- and out-of-plane critical magnetic fields show anisotropy. Scanning transmission electron microscopy images and X-ray reciprocal space mappings reveal that the RP bilayer nickelate films adopt a tetragonal phase under roughly 2% coherent epitaxial compressive strain in the NiO planes relative to the bulk. Our findings pave the way for comprehensive investigations of nickelate superconductors under ambient pressure conditions and for exploring superconductivity at higher transition temperatures through strain engineering in heterostructures.