Knowledge

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Based on the effective three-orbital model, this study systematically analyzes the interplay between multiorbital and nonlocal self-energy effects in the normal state of the high-pressure superconducting bilayer nickelate La₃Ni₂O₇ using the D-TRILEX many-body framework beyond dynamical mean-field theory. The results reveal that the low-energy physics is highly dependent on the interorbital interaction strength: when the interaction is weak, the renowned γ quasiparticle flat band lies below the Fermi level; as the interaction strengthens, this flat band crosses the Fermi level, causing electrons to scatter with ferromagnetic paramagnon excitations, thereby forming spin-polaron bound states. These bound states manifest as incoherent spectral weight shadow bands below the Fermi level. The findings unveil the existence of additional competing electronic states in bilayer nickelates, providing a theoretical basis for resolving recent controversies in angle-resolved photoemission spectroscopy experiments regarding spectral structures near the Fermi surface.

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Resonant inelastic X-ray scattering (RIXS) at the Ni L edge on single crystals of the trilayer nickelate La4Ni3O10 reveals collective spin excitations with a bandwidth of approximately 60 meV, comparable to that of the bilayer nickelate La3Ni2O7, but with significantly reduced spectral weight, indicating weaker electronic correlations in the trilayer system. Localized spin excitations at around 100 and 200 meV are also observed, originating from local dipole and quadrupole excitations. The dispersive magnetic excitations exhibit three-dimensional characteristics, and fitting with linear spin-wave theory yields comparable in-plane and out-of-plane exchange coupling parameters, with the interlayer coupling being the strongest. The results indicate that La4Ni3O10 possesses stronger three-dimensional magnetism, with its spin dynamics consistent with spin-density-wave order, while the reduced electronic correlations and three-dimensional multi-orbital character are key factors leading to differences in its magnetic excitation spectrum compared to the bilayer nickelate, providing important insights into the evolution of magnetism and its connection to superconductivity in the Ruddlesden-Popper nickelate family.

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Using polarization-resolved electronic Raman scattering, this study systematically maps the momentum-selective spin-density-wave (SDW) gap in the trilayer nickelate La4Ni3O10. The experiments reveal that SDW-induced spectral weight redistribution occurs simultaneously on the α pocket at the Brillouin zone center and on part of the β pocket near the zone boundary, with a corresponding gap energy of approximately 55 meV, whereas no comparable spectral weight suppression is observed in the diagonal region of the β pocket, indicating that this region remains nearly gapless. This momentum-space gap topology contrasts sharply with that of the bilayer nickelate La3Ni2O7, where only the β pocket exhibits an anisotropic SDW gap. These results establish distinct momentum-space gap topologies between bilayer and trilayer nickelates, providing new constraints on the ordering wave vector of the density-wave instability and the mechanism related to superconductivity.

Using Ni L3-edge resonant inelastic X-ray scattering (RIXS), this study compares low-energy spin excitations in the octahedral Ruddlesden-Popper (RP) phase Nd9Ni8O25 (non-superconducting) and its reduced planar phase Nd9Ni8O18 (exhibiting superconducting correlations at approximately 5 K). The results show that the octahedral phase exhibits a spin-density wave (SDW) ground state with an ordering wave vector of (1/4,1/4), where the low-energy spectrum is dominated by weakly dispersive paramagnons along the (0,π) and (π,π) directions; in contrast, the planar phase displays an elastic peak at (1/3,0) with dispersionless magnetic excitations at an energy of about 65 meV. Polarization-resolved RIXS further confirms the distinct nature of magnetic excitations in the two phases. These findings systematically reveal fundamental differences in the ground states and spin excitations between the two structural families, providing critical insights into the mechanism of nickelate superconductivity.

This study employs high-pressure synthesis and hydrostatic high-pressure transport techniques to systematically modulate the bandwidth and band filling in the bilayer nickelate La₃Ni₂O₇ family, aiming to investigate their effects on superconductivity and non-superconducting state properties. By partially substituting La with smaller Nd (which increases NiO₆ octahedral tilting and reduces bandwidth), the pressure required for the superconducting phase is significantly elevated; conversely, co-introducing Sr for hole doping reverses this trend, lowering the onset pressure of superconductivity. In the non-superconducting state, up to three characteristic resistance anomalies are observed, evolving with pressure, likely corresponding to charge density wave and spin density wave orders that compete with superconductivity. A comprehensive comparison of phase diagrams across samples with different compositions indicates that independent control of bandwidth and filling is key to unraveling the mechanism of unconventional superconductivity and its competing orders in this system.