Daily Overview: Today’s highlights focus on an in-depth understanding of the electronic structure of hybrid Ruddlesden–Popper nickelates. In [1], first-principles calculations reveal the coupling between chemical-pressure-driven structural phase transitions and the emergence of superconductivity in rare-earth-doped La₂RNi₂O₇ (R=Pr, Nd, Sm), along with the key characteristic that the d_(z²) orbital forms a hole-type Fermi surface at the Fermi level in the high-pressure tetragonal phase, and elucidate the importance of out-of-plane – in-plane orbital hybridization for superconducting pairing. In resonance, [2] systematically reviews the interlayer pairing picture of bilayer nickelates from a strong-correlation perspective, emphasizing the phase coherence provided by the hybridization of localized d_(z²) pairs with itinerant d_(x² − y²). Building on this, [3] employs a t − J model incorporating d_(z²) and d_(x² − y²) orbitals to deliver a unified superconducting theory covering both bulk and thin films, predicts distinct superconducting domes under electron/hole doping, and points out that weakening interlayer coupling could realize superconductivity at ambient pressure. Regarding the origin of magnetism, [4] proposes an itinerant spin-density-wave picture, in which mirror-selective interband nesting not only explains the small net magnetic moments and spin-wave-like excitations in La₃Ni₂O₇ and La₄Ni₃O₁₀, but also clarifies the intertwining of spin and charge orders. Furthermore, [5] uses resonant inelastic X‑ray scattering to observe plasmons for the first time in metallic Pr₄Ni₃O₈; comparison with cuprates reveals weaker electronic transitions and enhanced screening effects in the nickelates, providing direct experimental evidence to quantitatively define the key energy scales of superconductivity. arXiv submission processing window: 2026-06-16 00:00 to 2026-06-16 00:00 UTC.

1. Electronic structure trends in La$_{2}R$Ni$_2$O$_7$ ($R=$ Pr, Nd, Sm) from first-principles

Summary: Using first-principles calculations, this study systematically investigates the crystal structures and electronic properties of rare-earth-doped bilayer Ruddlesden-Popper nickelates La₂RNi₂O₇ (R = Pr, Nd, Sm). The calculations show that dopant atoms preferentially occupy the La site in the rock-salt layer, and from Pr to Sm, the gradual decrease in ionic radius induces a chemical pressure effect that progressively reduces the unit-cell volume. All systems undergo a pressure-driven phase transition from a monoclinic to a tetragonal structure, with the transition pressure increasing as the dopant ion size decreases, and this transformation roughly coincides with the emergence of superconductivity. The high-pressure tetragonal phase is characterized by a flat d_(z^2) band crossing the Fermi level at the Γ point, creating an additional hole-type Fermi surface pocket. Although the doping-induced changes in electronic structure are subtle, with decreasing R ion radius the dominant in-plane hopping integrals are overall enhanced, whereas the interlayer hopping weakens due to the shortening of the apical Ni–O(rock-salt) bond length (despite the compressed c-axis). These hopping trends reveal the importance of hybridization between out-of-plane orbitals and in-plane itinerant states for superconducting pairing, offering new microscopic insights into how R doping controls the superconducting transition temperature.

2. Interlayer pairing mechanism for bilayer nickelate superconductors

Summary: This review focuses on the interlayer pairing mechanism in bilayer nickelate superconductors, distilling key physical elements from experimental observations, including the hybridized electronic structure of Ni-3d_{x^2-y^2} and 3d_{z^2}, orbitally dependent electron correlations, Hund’s coupling, and strong interlayer magnetic coupling. Introducing a bilayer two-orbital Hubbard model and its simplified t‑J variant, the interlayer pairing theory within the strong-correlation framework is systematically expounded by starting from the interlayer valence bond picture in the atomic limit of the half‑filled d_{z^2} orbital, with special emphasis on the hybridization mechanism in which local singlet pairing of d_{z^2} supplies the pairing energy and hybridization of that pairing with itinerant d_{x^2-y^2} orbitals promotes superconducting phase coherence. The analysis also addresses pairing symmetry and the dependence of the superconducting transition temperature Tc on internal and external parameters such as pressure and oxygen content, and discusses normal‑state behaviors—Fermi‑liquid, non‑Fermi‑liquid, weakly insulating, and pseudogap—as well as effects like Kondo scattering induced by oxygen vacancies, before briefly touching upon weak‑coupling theories based on spin fluctuations arising from Fermi‑surface nesting.

3. A unified theory of thin film and bulk bilayer nickelates

Summary: This paper proposes a unified theory based on a two-component scenario, employing a t-J model that includes both d_{x^2-y^2} and d_{z^2} orbitals, and systematically investigates the superconducting behavior of bilayer nickelates in pressurized bulk and thin films by self-consistently solving the self-energy and superconducting pairing vertex within a particle representation. The theory predicts that, when the interlayer superexchange coupling J⊥ is strong, electron and hole doping each form a distinct superconducting dome separated by a valence-bond state near half-filling of the d_{z^2} orbital, whereas for weak or moderate J⊥ the two domes merge into a single low-Tc superconducting region spanning half-filling. Increasing doping drives the normal state from Fermi liquid to non-Fermi liquid or weak insulating behavior, with a quasilinear temperature-dependent scattering rate appearing near optimal Tc; oxygen vacancies or chemical substitutions disrupt the interlayer valence bonds, suppressing superconductivity and inducing localized Kondo scattering of d_{x^2-y^2} electrons. These results provide a unified explanation for differences in superconducting transition and normal-state properties between bulk and thin films, the doping effect of the d_{z^2} band, as well as Kondo resistance minima and negative magnetoresistance observed in non-superconducting samples. The theory further indicates that bulk superconductivity can be achieved at ambient pressure by doping or weakening interlayer magnetic coupling, and predicts that electron doping will yield higher Tc.

4. Itinerant Nature of Spin-Density-Wave Order in Ruddlesden-Popper Nickelates

Summary: This study addresses the magnetic origin of layered Ruddlesden–Popper nickelates La₃Ni₂O₇ and La₄Ni₃O₁₀ by proposing a unified itinerant-electron description. The core approach exploits mirror symmetry of the NiO₂ multilayer structure, decomposing low-energy electronic states into mirror-even and mirror-odd parity sectors; it is found that nesting between bands of opposite mirror parity drives a mirror-selective itinerant spin-density-wave instability. The resulting collective excitation modes naturally reproduce the experimentally observed spin-wave-like magnetic excitation spectra, and the small ordered magnetic moment arises from Fermi surface reconstruction rather than local moments. For La₄Ni₃O₁₀, this spin-density wave further induces a secondary mirror-even charge-density wave, intertwining spin and charge orders. The results indicate that magnetism in multilayer nickelates is inherently itinerant, and the mirror-selective interband spin-density-wave order is the key principle for a unified understanding of their magnetic correlations, with the effective exchange coupling not being a direct measure of microscopic superexchange.

5. Observation of correlated plasmons in low-valence nickelates

Summary: This study utilizes resonant inelastic X-ray scattering (RIXS) to observe plasmons for the first time in the metallic low-nickel-valence nickelate Pr₄Ni₃O₈ and performs a systematic comparison with the overdoped cuprate La₂₋ₓSrₓCuO₄. The plasmons in the nickelate exhibit larger damping and lower velocity, originating from weakened electron hopping and enhanced screening of long-range Coulomb interactions. Random phase approximation (RPA) calculations reproduce the experimental observations, confirming significantly reduced in-plane hopping and Coulomb interactions in the nickelate. With increasing temperature, the plasmon in Pr₄Ni₃O₈ softens and its intensity decreases, while the plasmon energy in the cuprate remains nearly unchanged but its broadening increases, revealing distinct charge screening scenarios in the two materials. Furthermore, the nickelate shows no clear out-of-plane plasmon dispersion, which may be related to oscillation modes in its trilayer structure. These findings uncover essential differences in charge dynamics between nickelates and cuprates, offering quantitative experimental benchmarks for determining key parameters of unconventional superconductivity.