Critical Thickness and Long‐Term Ambient Stability in Superconducting LaPr ₂ Ni ₂ O ₇ Films

Abstract The recent observation of ambient‐pressure superconductivity in compressively strained (La,Pr) 3 Ni 2 O 7 films marks a significant advance in nickelate superconductivity research. However, their fabrication remains challenging, with reported thickness limited to <6.6 nm and pronounced ambient degradation. In this study, LaPr 2 Ni 2 O 7 films with nominal thicknesses ranging from 3.5 to 23.5 nm are fabricated. Superconductivity is observed in all samples, with a maximum onset transition temperature ( T c ) of 44 K. No systematic correlation between T c and film thickness is identified. Angle‐dependent T c measurements under external magnetic fields and vortex anisotropy analysis indicate 2D superconductivity in all samples. Structural and transport measurements show that superconductivity in LaPr 2 Ni 2 O 7 is confined to within 10 nm of the interface, while thicker films develop a protective (La,Pr) 4 Ni 3 O 10 surface layer that enhances stability. Ex situ amorphous oxide capping layers further suppress superconducting degradation, yielding 10‐fold stability enhancement in ultrathin films (3 ≈ 4 nm) and prolonging stability from 30 to more than 100 days in thicker films.