Critical role of hydrogen for superconductivity in nickelates

The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized by a topotactic reaction with metal hydrides¹. This method changes the nickelates from the perovskite to an infinite-layer structure by deintercalation of apical oxygens^1-3. Such a chemical reaction may introduce hydrogen (H), influencing the physical properties of the end materials^4-9. Unfortunately, H is insensitive to most characterization techniques and is difficult to detect because of its light weight. Here, in optimally Sr doped Nd_0.8Sr_0.2NiO₂H epitaxial films, secondary-ion mass spectroscopy shows abundant H existing in the form of Nd_0.8Sr_0.2NiO₂H_x (x ≅ 0.2-0.5). Zero resistivity is found within a very narrow H-doping window of 0.22 ≤ x ≤ 0.28, showing unequivocally the critical role of H in superconductivity. Resonant inelastic X-ray scattering demonstrates the existence of itinerant interstitial s (IIS) orbitals originating from apical oxygen deintercalation. Density functional theory calculations show that electronegative H^- occupies the apical oxygen sites annihilating IIS orbitals, reducing the IIS-Ni 3d orbital hybridization. This leads the electronic structure of H-doped Nd_0.8Sr_0.2NiO₂H_x to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for superconductivity in epitaxial infinite-layer nickelates.