Optimization of Infinite-Layer Nickelate Superconductors via Three In Situ Atomic Hydrogen Reduction Methods

Infinite-layer nickelates, obtained by removing the apical oxygen from perovskite precursors, are the first nickelate system to exhibit superconductivity and provide a platform for exploring unconventional superconductivity. Although the traditional CaH₂ sealed-tube reduction method is simple and effective, it is an ex-situ process that tends to cause surface contamination or degradation, making it unsuitable for surfacesensitive measurements like ARPES. To address this issue, we established three different in-situ atomic hydrogen reduction methods in an ultrahigh vacuum chamber-namely, a lab-based RF plasma cracker, an industrial RF plasma cracker, and a thermal gas cracker. Comprehensive optimization of key parameters-including hydrogen flow, RF power or filament temperature, reduction temperature, and time-was carried out for each method. Structural evolution was monitored by X-ray diffraction (XRD), surface morphology was characterized by atomic force microscopy (AFM), and superconducting properties were examined through electrical transport measurements. The results demonstrate that all three in-situ methods can achieve reduction and superconducting properties comparable to or better than CaH₂ reduction. Moreover, all atomic hydrogen approaches yield lower surface roughness than CaH₂ from the same precursor, highlighting their clear advantage in enhancing surface flatness. Notably, the industrial RF plasma source, owing to its higher hydrogen production efficiency, enables sufficient reduction under milder conditions, resulting in even smoother surfaces. This study also provides a detailed summary of the parameter optimization for each method, offering valuable guidance for the controlled reduction of high-quality infinite-layer nickelate thin films.