Critical Thickness and Long-Term Ambient Stability in Superconducting LaPr₂Ni₂O₇ Films

The recent observation of ambient-pressure superconductivity in compressively strained (La,Pr)₃Ni₂O₇ 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, LaPr₂Ni₂O₇ 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 LaPr₂Ni₂O₇ is confined to within 10 nm of the interface, while thicker films develop a protective (La,Pr)₄Ni₃O₁₀ 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.