Unlocking Superconductivity in NdFeAsO via Uranium-Mediated Electron Doping

NdFeAsO, a typical 1111-type iron-based superconductor (IBS), has a layered crystal structure consisting of alternating charge-reservoir Nd₂O₂ layers and conducting Fe₂As₂ layers, offering substantial potential for carrier doping. In this study, uranium (U) was incorporated into the Nd site, and a series of Nd_1-xU_xFeAsO samples were successfully synthesized. Rietveld refinement of X-ray diffraction (XRD) patterns revealed a systematic lattice contraction with increasing U content. Consistently, X-ray photoelectron spectroscopy (XPS) directly confirmed the substitution of U⁴⁺ for Nd³⁺ and the electron-doping nature of this substitution. Electrical transport measurements demonstrated that U doping effectively suppresses the spin-density wave (SDW) transition of the parent compound at ∼150 K and induces bulk superconductivity in the doping range 0.15 ≤ x ≤ 0.25. The optimally doped sample (x = 0.25) exhibits a high superconducting critical temperature (T_c) of 47.5 K, and its superconducting onset transition is markedly insensitive to the applied magnetic field, suggesting the presence of a high upper critical field (H_c2). This work demonstrates that U doping offers an alternative route for achieving superconductivity in the 1111-family IBS.