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Using density functional theory, we calculate the electronic structure of the bilayer infinite-layer nickelate La₃Ni₂O₅F, revealing its ideal two-dimensional character and the coexistence of two distinct quasiparticle behaviors. After treating the oxygen/fluorine disorder with the virtual crystal approximation, band structure calculations show that the conventional Ni dpσ band forms a hole-like Fermi surface, whereas an E* band originating from interstitial density gives rise to a cylindrical electron Fermi surface, resulting in a self-doping of 0.18 electrons. This interstitial density is distributed between the La layers that lack apical oxygen, and as the Ni dₓz/dyz bands approach the M point with parallel linear dispersion, they couple with it to form a nearly non-analytic Dirac point, exhibiting an exotic interstitial-orbital band coupling effect. Concurrently, the d_z² band undergoes a symmetry-driven splitting of approximately 1 eV through interaction with the interstitial density. This dual electron–hole character is expected to govern normal-state transport and far-infrared properties, may influence the superconducting state of nickelates, and offers a fresh perspective for understanding the physics of infinite-layer nickelates.

This study investigates the dimensionality of the superconducting state in infinite-layer nickel oxides by mapping the vortex phase diagram of superconducting Pr0.8Sr0.2NiO2 thin films from multiple perspectives. Experimental results reveal that low-disorder films exhibit a quasi-two-dimensional vortex liquid-to-glass transition, while increasing disorder drives the system into a pure two-dimensional state. This finding indicates that pure two-dimensionality is not an intrinsic property but an extrinsic phenomenon caused by the decoupling of NiO2 layers due to enhanced disorder. The work establishes disorder as a key tuning parameter for superconductivity in infinite-layer nickel oxides and identifies that disorder primarily resides within the NiO2 layers, offering two fundamental insights for understanding this class of materials.

Disorder-Induced Suppression of Superconductivity in Infinite-Layer Nickelates

This study employs resonant inelastic X-ray scattering (RIXS) to directly compare the electronic and magnetic excitation properties of trilayer nickelate La₄Ni₃O₁₀ and bilayer La₃Ni₂O₇. The results show that La₄Ni₃O₁₀ exhibits more itinerant behavior, evidenced by broader Ni d-d orbital excitations and a stronger fluorescence background, indicating weaker electronic correlations than in the bilayer system. Despite the weaker correlations, clear collective spin excitations are observed, including dispersive acoustic and optical magnon branches as well as incommensurate spin density waves (SDW). Using linear spin-wave theory analysis, the interlayer superexchange interaction Jz is extracted to be approximately 22 meV, significantly smaller than that in La₃Ni₂O₇. The weaker electron correlations and reduced interlayer magnetic exchange together account for the substantially lower superconducting transition temperature of the trilayer compound (about 30 K) compared to the bilayer (about 80 K). This study establishes interlayer magnetic coupling and electronic correlations as key parameters for superconductivity in layered nickelates, providing important constraints for understanding the superconducting pairing mechanism in this emerging family.

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