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We performed a systematic comparison of the two polytypes of La3Ni2O7—the alternating monolayer-trilayer LNO-1313 and the bilayer LNO-2222—and the trilayer compound La4Ni3O10 using Ni L3-edge and O K-edge resonant inelastic X-ray scattering (RIXS). We find that the electronic, magnetic, and lattice excitations of LNO-1313 and La4Ni3O10 are highly similar, whereas bilayer LNO-2222 exhibits distinctly different features. Compared with LNO-2222, LNO-1313 and La4Ni3O10 possess weaker orbital polarization, enhanced 3d^8 L (ligand-hole) character, a smaller out-of-plane magnetic exchange scale, and stronger electron-phonon coupling. Within an effective local-moment framework, the spin excitations arising from strong antiferromagnetic interlayer coupling can be naturally described by an entangled-dimer picture, whose advantage is most evident in bilayer LNO-2222, where the interlayer coupling exceeds the intralayer interactions. These results provide key experimental constraints for theoretical models of the low-energy physics relevant to superconductivity in layered nickelates.

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This study employs density functional theory combined with dynamical mean-field theory (DFT+DMFT) to systematically analyze the layer-resolved electronic structure and correlation effects in low-n tetragonal nickelates. The results reveal that the electronic correlation strength of Ni-d orbitals in undoped systems increases with layer number, and that the inner NiO₂ planes consistently exhibit stronger correlations than the outer ones, a discrepancy originating from the inhomogeneous spatial charge distribution. For the n=2 and n=3 compounds, which are non-superconducting due to excessive hole doping, an electronic compensation strategy via Cl substitution at spacer-layer oxygen sites is proposed, and virtual crystal approximation simulations tune the nominal Ni valence to match that of the optimally superconducting n=6 system. Calculations demonstrate that Cl doping significantly enhances the Ni-d mass enhancement factor in the low-layer-number systems, driving them into the strongly correlated metallic regime while preserving the low-energy electronic structure. This work highlights the critical role of layer-resolved electronic correlations in the superconductivity mechanism and predicts that spacer-layer Cl doping is a viable pathway to convert low-n tetragonal nickelates into potential superconducting candidates.

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This study employs a multi-orbital itinerant electron framework, combined with Hartree-Fock and random phase approximation methods, to systematically analyze the magnetic ground states and transverse spin excitations of bilayer and trilayer nickelates. For the bilayer system, although the double-stripe order has slightly lower energy, the excitation spectrum of the single-stripe state exhibits anisotropic low-energy cone-shaped dispersion at the wavevector Q_BL and isotropic high-energy excitations near the Γ point, showing qualitative consistency with resonant inelastic X-ray scattering and neutron scattering experiments; it is also found that the energy of the mirror-even interlayer optical mode at Q_BL coincides with that of the mirror-odd mode at Γ. In the trilayer system, both mirror-odd and mirror-even spin-density wave orders can be stabilized, with the mirror-odd state having lower energy and hosting a near-zero-gap mode predominantly from the middle layer, whereas the mirror-even state supports only one acoustic mode and two gapped optical modes; comparison with experimental data supports the mirror-odd order picture. The results demonstrate that magnetic excitations can serve as a sensitive probe to distinguish magnetic order configurations and reinforce the conclusion that the magnetism in multilayer nickelates shares a common itinerant origin.

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