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Based on experimental evidence of orbital-selective electronic correlations, this work proposes a new magnetic description framework for bilayer nickelate La₃Ni₂O₇. In the bad-metal regime of the normal state, the system lies close to an orbital-selective Mott phase, with the electronic spectrum decomposing into coherent d_(x²-y²) quasiparticles and incoherent d_(z²) local moments. An effective spin model incorporating superexchange between local moments and RKKY interactions mediated by coherent electrons is constructed, where the RKKY contribution dominates third-neighbor coupling and introduces magnetic frustration. The model naturally stabilizes a noncoplanar antiferromagnetic ground state with a wavevector near (π/2, π/2) and antiferromagnetic interlayer stacking, consistent with neutron scattering experiments. The calculated spin-wave spectrum contains low-frequency acoustic and high-frequency optical branches, with the acoustic branch softening at the ordering wavevector and an overall bandwidth of about 80 meV, in agreement with resonant inelastic X-ray scattering data. These results reveal that orbital-selective correlations are the essential ingredient determining magnetism in bilayer nickelates and indicate that low-energy magnetic fluctuations and short-range exchange interactions may provide the pairing glue for unconventional superconductivity.

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Using angle-resolved photoemission spectroscopy (ARPES) with an optimized surface treatment, we probe the low-energy electronic structure of the infinite-layer nickelate superconductor La0.8Ca0.2NiO2 and its parent LaNiO2 thin films, resolving clear quasiparticle peaks for the first time in this system. In La0.8Ca0.2NiO2, the imaginary part of the electron self-energy exhibits a roughly linear dependence on energy with a slope that progressively increases from the (π/2, π/2) to the (π, 0) direction, revealing momentum-dependent marginal-Fermi-liquid-like scattering. Both films display low-energy spectral weight that is gradually suppressed from the diagonal toward the antinodal region, with stronger suppression in the parent compound; however, finite Fermi-level spectral weight persists over the entire Fermi surface, and no leading-edge shift or back-bending is observed, indicating the absence of the pseudogap typical of cuprates. These findings demonstrate that momentum-selective correlations and marginal-Fermi-liquid-like scattering can emerge without a detectable pseudogap, providing a crucial benchmark for identifying the essential electronic ingredients of the high-Tc normal state and suggesting that the pseudogap is not a requirement, while charge-transfer energy and the orbital character of doped carriers may play decisive roles.

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9 linked papers