Critical Role of Hydrogen for Superconductivity in Infinite-layer Nickelates

The newly discovered nickelate superconductors so far only exist in epitaxial thin films synthesized via topotactic reaction with metal hydrides1. This method changes the nickelates from the perovskite to the infinite-layer structure by deintercalation of apical oxygens1-3. Such chemical reaction may introduce hydrogen (H) influencing the physical properties of the end materials4-9. Unfortunately, H is insensitive to most of the characterization techniques and is difficult to detect due to its light weight. Here, in the optimally doped Nd0.8Sr0.2NiO2H epitaxial film, secondary-ion mass spectroscopy reveals abundant H existing in the form of Nd0.8Sr0.2NiO2Hx (x = 0.2 ~ 0.5). Remarkably, 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) orbital originating from apical oxygen deintercalation. Density function 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 Nd0.8Sr0.2NiO2Hx to be more two-dimensional-like, which might be relevant for the observed superconductivity. We highlight that H is an important ingredient for the superconductivity in epitaxial infinite-layer nickelates.