Superconductivity in Sr-doped La₃Ni₂O₇ thin films

Recent studies have demonstrated ambient pressure superconductivity in compressively strained La₃Ni₂O₇ thin films, yet the phase diagram of heterovalent doping-critical for advancing the field-remains underexplored. Here we report superconductivity in Sr²⁺-doped La_3-xSr_xNi₂O₇ films. The superconducting transition temperature (T_c) follows an incomplete dome-like profile, maintaining similar T_c values across a wide doping range (0 ≤ x ≤ 0.21) before diminishing near x ≈ 0.38. Optimally doped films achieve a T_c value of ~42 K, with a high critical current (J_c > 1.4 kA cm^-2 at 2 K) and upper critical fields (μ₀H_c,∥(0) = 83.7 T, μ₀H_c,⟂(0) = 110.3 T). Scanning transmission electron microscopy reveals that oxygen vacancies predominantly occupy planar NiO₂ sites-unlike apical-site vacancies in bulk samples-due to compressive strain. Additionally, the elongated out-of-plane Ni-O bonds, exceeding those in pressurized bulk samples by 4%, likely weaken the interlayer $d_{z^2}$ coupling and contribute to the reduced T_c in strained films.