[PHYSICAL REVIEW LETTERS 133, 066503 (2024) – Editors’ Suggestion]
A spectroscopic study of the infinite-layer Nd0.8Sr0.2NiO2 thin films unveils a singular Ni 3d orbital in an optimally reduced condition, highlighting the importance of topotactic reduction for achieving superconductivity.
Topotactic reduction is a unique soft chemistry process that removes the apical oxygen from the oxygen-metal octahedron, transforming transition metal oxides from a conventional perovskite structure to an infinite-layer structure. This process is believed to be a prerequisite for achieving superconductivity in the newly discovered infinite-layer nickelates. However, its effect on atomic reconstruction and electronic structures remains largely unresolved. Here, we report on ion mass spectrometry and X-ray absorption spectroscopy measurements of infinite-layer Nd0.8Sr0.2NiO2 thin films with varying levels of topotactic reduction. Contrary to previous beliefs, we found that reduction-induced hydrogen intercalation is negligible in the pure infinite-layer phase and is not critical to superconductivity. Instead, we observed a singular Ni 3d orbital in the optimally reduced state and orbital mixing in both the under-reduced and over-reduced states. Our findings reveal the critical role of reduction in modulating the Ni 3d orbital polarization and its impact on the superconducting properties, providing new insights into the mechanisms of superconductivity in infinite-layer nickelates.