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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.

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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.

1 linked paper

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.

This study proposes a unified theory based on a two-component model to explain a series of key experimental phenomena in pressurized bulk and thin-film bilayer nickelate superconductors. Centered on the interlayer superexchange coupling and hybridization between strongly correlated localized electrons and itinerant electrons at the nickel orbitals, the theory predicts two distinct behaviors of the superconducting transition temperature with doping: when the interlayer superexchange coupling is strong, electron or hole doping respectively produce two superconducting domes, with a non-superconducting interlayer valence-bond state appearing near half-filling; when the coupling is weak or moderate, the two domes merge into a single dome that spans half-filling but has a lower maximum temperature. Increasing doping drives the normal state from a Fermi liquid to a non-Fermi liquid or weakly insulating state, with a quasi-linear resistivity scattering rate emerging near optimal doping. Oxygen vacancies or chemical substitutions can disrupt the interlayer valence bond, simultaneously suppressing superconductivity and inducing local Kondo scattering of itinerant electrons, which explains the logarithmic temperature dependence of resistivity and the negative magnetoresistance observed in non-superconducting samples. This framework uniformly accounts for the differences in superconducting transition and normal state between bulk and thin films, the effects of hole doping and oxygen stoichiometry on the dome shape, and the competitive relationship between superconductivity and the Kondo effect. Based on the theory, the authors propose that bulk superconductivity at ambient pressure can be achieved through doping or by reducing the interlayer magnetic coupling, and predict that electron doping will yield higher transition temperatures.

Based on the previously proposed effective d_{x^2-y^2} orbital bilayer t-J∥-J⊥ model with model parameters input from first-principles calculations, this paper provides a unified explanation for a series of experiments on the regulation of the superconducting transition temperature (Tc) in La₃Ni₂O₇ via oxygen stoichiometry, elemental substitution, pressure, or strain, using slave-boson mean-field and density matrix renormalization group methods. The model reveals that, due to the near quarter-filling of the d_{x^2-y^2} orbital, its Tc tuning behavior resembles that of hole-doped overdoped cuprates. In terms of doping dependence, the system exhibits particle-hole asymmetry: hole doping suppresses Tc by making the system more overdoped, while electron doping has the opposite effect, explaining the Tc suppression caused by excess oxygen or Ca/Sr substitution for La, as well as the “half-dome” behavior in oxygen stoichiometry tuning. Regarding interaction dependence, Tc varies with the interlayer antiferromagnetic superexchange interaction J⊥, accounting for the enhancement of bulk Tc by Sm/Nd substitution for La, the “right-triangle” shape of pressure-dependent bulk Tc, and the enhancement of Tc under compressive strain in thin films. Compared with weak-coupling theory (where Tc depends mainly on the density of states) and the d_{z^2} orbital-dominated pairing mechanism (where Tc is proportional to the d_{z^2} hole density), this model provides a more natural and unified explanation. The paper further proposes that Tc can be increased through electron doping that does not introduce disorder, such as substituting La with higher-valent elements.

This study employs superconducting density functional theory to perform ab initio calculations on optimally doped infinite-layer nickelates Re₀.₈Sr₀.₂NiO₂ (Re=La, Pr, Nd), treating electron–phonon coupling, screened Coulomb repulsion, and spin fluctuations on an equal footing. The results reveal that these materials are two-band superconductors exhibiting opposite-sign d_(x²−y²)-wave pairing gaps on different Fermi surfaces; when spin fluctuations are switched off, the critical temperature drops to a negligible ~0.01 K, indicating that superconductivity is driven by spin fluctuations. On the large quasi-two-dimensional Fermi surface at the Brillouin zone center, the spin fluctuation strength is an order of magnitude larger than that of electron–phonon coupling and Coulomb repulsion, thereby dominating the pairing mechanism, whereas electron–phonon coupling plays the principal role on the small three-dimensional electron pockets at the zone corners. The nodal d-wave gap structure originates from a pronounced peak of the Lindhard response function at the zone corners. The calculated Fermi surfaces, critical temperature, nodal gaps, and quasiparticle density of states are in agreement with most experimental observations, and the predicted unconventional superconducting properties such as scanning tunneling spectra await direct experimental verification.

1 linked paper