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This study utilized X-ray absorption spectroscopy and resonant inelastic X-ray scattering to independently control the strain and oxygen content of (La,Pr)₃Ni₂O₇₋δ thin films, tracking the microscopic evolution from a non-superconducting parent phase to a superconducting phase. The results demonstrate that both tuning methods induce delocalization of the oxygen 2p_z and nickel 3d_z² orbitals, as evidenced by spectral weight transfer from the “upper Hubbard” peak to the hole peak in the O K-edge absorption spectra, accompanied by broadening and weakening of the Ni L-edge absorption spectra and dd excitations. Concurrently, the intensity and correlation length of the long-range spin density wave (SDW) order are significantly suppressed, indicating direct competition with superconductivity; while short-range magnons are damped, their bandwidth remains unchanged. This suggests that the delocalization of oxygen 2p_z and nickel 3d_z² orbitals, along with the robustness of short-range magnons during the melting of the SDW order, are prerequisites for achieving superconductivity, thus providing constraints for theoretical models and pointing toward an orbital-selective pathway for designing nickel-based superconductors.

This study developed a megapascal-level high-pressure oxygen-assisted chemical synthesis route that successfully enabled the effective growth of infinite-layer nickelates extending to heavier rare earth elements, with composition (RE₁₋ᵧRE’ᵧ)₁₋ₓEuₓNiO₂ (RE/RE’: Pr, Nd, Sm, Gd, Dy). Hole doping was realized through Eu²⁺/Eu³⁺ valence variation. At the superconducting dome boundaries of Nd₁₋ₓEuₓNiO₂ and Pr₁₋ₓEuₓNiO₂ systems, robust uniaxial anisotropic magnetic-field reentrant superconductivity was observed, arising from the competition between Eu²⁺ 4f⁷ magnetic moments and magnetic fluctuations, while the optimally doped regions exhibited conventional high-temperature superconductivity with a critical current density reaching ~266 kA/cm² at 2 K, surpassing that of traditional Sr/Ca-doped systems. Further introduction of different RE′ magnetic ions allowed tuning of the exchange field strength and thereby modulation of quantum criticality, both enhancing the reentrant behavior and elevating T_c to 40.1 K. This work reveals the critical role of rare-earth 4f magnetic moments in modulating pairing strength and quantum criticality, establishing a synthetic foundation for utilizing the infinite-layer nickelate platform to investigate 4f-related unconventional superconductivity and quantum phase transitions.

This study finds that zero resistance and a diamagnetic response are observed in stoichiometric PrNiO₂ thin films without cation substitution and in the absence of a capping layer, thereby confirming the intrinsic superconductivity of the parent infinite-layer nickelate. Heterostructure engineering by inserting a non-superconducting buffer layer rules out contributions from substrate interface effects. Isovalent trivalent La substitution likewise preserves superconductivity, demonstrating that the phenomenon is not unique to PrNiO₂. In contrast, trace divalent Sr or Ca substitution (within about 3%) rapidly suppresses superconductivity so that it no longer appears. Combined with angle-resolved photoemission spectroscopy measurements, this reveals that the superconducting phase is confined to an extremely narrow hole-doping range within 3% beyond the parent PrNiO₂. A non-superconducting region exists between this phase and the previously established superconducting dome at approximately 20% divalent doping, giving rise to two separated superconducting regions on the phase diagram. Furthermore, the parent superconducting phase exhibits a significantly stronger upper critical field anisotropy. These results indicate that infinite-layer nickelates are not simply cuprate analogs but possess unique superconducting physics and segregated superconducting domains.

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In compressively strained bilayer nickelate thin films, by continuously tuning the oxygen stoichiometry, researchers have discovered a superconducting half-dome. Starting from the optimal superconducting state, increasing the oxygen content progressively suppresses superconductivity, driving a transition toward a metallic phase; conversely, decreasing the oxygen content induces a granular superconductor-insulator transition while the onset superconducting temperature remains unchanged. This half-dome structure originates from the distinct roles of interstitial oxygen and oxygen vacancies: the former primarily regulates carrier concentration through doping effects, whereas the latter introduces strong scattering that leads to electronic inhomogeneity. Experiments show that this half-dome consistently appears across different rare-earth combinations and with or without alkaline-earth doping, revealing a universal feature of the bilayer nickelate phase diagram. This finding offers new perspectives for understanding the emergence and suppression of superconductivity in correlated electron systems.