Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of mixed Ruddlesden-Popper nickelates. In [1], the superconducting pairing in bilayer nickelate La₃Ni₂O₇ is revealed to have a hierarchical structure, with the primary pairing originating from the bonding-antibonding splitting of the Ni 3d({z^2}) orbitals, while orbital hybridization redistributes the superconducting correlations to the 3d({x^2-y^2}) channel. The resulting s± state is robust against changes in Fermi surface topology, unifying previously divergent theoretical perspectives. [2] unifies a series of experiments on the superconducting transition temperature of La₃Ni₂O₇ under oxygen stoichiometry, elemental substitution, pressure, and strain modulation, based on an effective d(_{x^2-y^2}) orbital bilayer t-J model. It is found that the Tc behavior resembles that of overdoped cuprates and exhibits particle-hole asymmetry, and it is proposed that electron doping can further enhance Tc. [3] focuses on the trilayer La₄Ni₃O₁₀, employing cluster exact diagonalization to reveal pressure-induced redistribution of oxygen hole states: under ambient pressure, excess holes are localized in the central NiO₂ layer and interlayer O p_z orbitals, while under high pressure they redistribute to the outer layers, emphasizing similarities with bilayer nickelates and proposing possible charge and spin ordered states. arXiv submission processing window: 2026-03-17 00:00 to 2026-03-17 00:00 UTC.

1. Hierarchical structure of primary and hybridization-induced superconducting correlations in bilayer nickelates

Summary: This study employs the variational Monte Carlo method to perform nonperturbative calculations on a bilayer two-orbital Hubbard model, revealing the hierarchical structure of superconducting pairing in the bilayer layered nickelate La₃Ni₂O₇. It is found that the primary pairing interaction originates from the bonding-anti-bonding splitting of the Ni 3d({z^2}) orbital, while orbital hybridization redistributes superconducting correlations into the 3d({x^2-y^2}) channel, despite its intrinsically weak pairing interaction. This distinction between the origin of pairing and the source of superconducting correlations explains why both orbital channels exhibit comparable long-range superconducting correlations, and the resulting s± state is robust against changes in Fermi surface topology, such as the disappearance of the α Fermi pocket. The results reconcile previously divergent theoretical perspectives on the pairing mechanism, indicating that the strength of superconducting correlations is primarily determined by the orbital character of the low-energy density of states, whereas the pairing interaction stems from orbital level splitting in the bilayer structure, highlighting the crucial role of orbital hybridization in stabilizing superconductivity in multilayer layered superconductors.

2. A Unified Understanding of the Experimental Controlling of the T$_\text{c}$ of La$_3$Ni$_2$O$_7$

Summary: 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.

3. Pressure induced redistribution of oxygen hole states in La$_{4}$Ni$_{3}$O$_{10}$

Summary: Using density functional calculations and a multi-orbital, multi-atom cluster exact diagonalization method incorporating local exchange and Coulomb interactions, this work explores the local low-energy electronic states of trilayer La₄Ni₃O₁₀ with a minimal Ni₃O₁₄ cluster. Under ambient pressure, when all three Ni ions are nominally in the +2 valence state, one of the two extra holes is localized in the central NiO₂ layer, forming a Zhang-Rice singlet with the d_{x²-y²} orbital, while the other hole primarily occupies the antibonding combination of the two interlayer O p_z orbitals and subsequently hybridizes with the out-of-plane three-spin polaron formed by the d_{z²} orbitals of the three NiO₂ layers. In this regime, in-plane spin alternation is carried by the outer-layer d_{x²-y²} orbitals, interlayer antiferromagnetic correlations are present, and the central layer is insulating with negligible magnetic moment. Under high pressure, however, the two extra holes become concentrated in one outer layer and the inner layer, respectively, forming either a Zhang-Rice singlet with the d_{x²-y²} orbital or an in-plane three-spin polaron with adjacent clusters. This study emphasizes the similarity between bilayer La₃Ni₂O₇ and trilayer La₄Ni₃O₁₀, and proposes possible charge and spin ordered states based on the cluster results.