Superconductivity and phase diagram 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 unexplored. Here, we report superconductivity in Sr²⁺-doped La_3-xSr_xNi₂O₇ films synthesized via molecular beam epitaxy with ozone-assisted post-annealing. The superconducting transition temperature (T_c) follows an incomplete dome-like profile, maintaining T_c values similar to the undoped sample across a wide doping range (0 ≤ x ≤ 0.21) before diminishing at x ≈ 0.38. Optimally doped films (x = 0.09) achieve T_c of ~ 42 K, with high critical current (J_c > 1.4 kA/cm² at 2 K) and upper critical fields (μ₀H_c,‖(0) = 83.7 T, μ₀H_c,⊥(0) = 110.3 T), comparable to reported La_3-xPr_xNi₂O₇ films. Scanning transmission electron microscopy reveals oxygen vacancies predominantly occupy at planar NiO₂ sites—unlike apical-site vacancies in bulk samples—due to Coulomb repulsion destabilizing planar oxygen under compressive strain. Additionally, the elongated out-of-plane Ni–O bonds, exceeding those in pressurized bulk samples by 4%, likely weaken the interlayer d_z² coupling, thus contributing to the reduced T_c in strained films. This work establishes heterovalent Sr²⁺ doping as a robust tuning parameter for nickelate superconductivity, unveiling a unique phase diagram topology.