Paper

Lattice-Charge Coupling in a Trilayer Nickelate with Intertwined Density Wave Order

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.

Based on a first-principles-derived two-orbital model, this study investigates the mechanism of topological superconductivity in substrate-supported bilayer nickelate thin films (La₃Ni₂O₇). Under the assumption of dominant interlayer pairing, the substrate-induced layer-asymmetric potential and layer-staggered Rashba spin-orbit coupling drive the system into a time-reversal-invariant topological superconducting phase of class DIII, which is protected by diagonal mirror symmetry. This nontrivial topology originates from the reversal of the pairing sign on the spin-split bonding Fermi surface, manifesting as a nonzero diagonal mirror winding number and Majorana Kramers pairs at the [11] edge. Furthermore, orbital hybridization gives rise to bulk Dirac nodes and zero-energy flat bands that deviate from the diagonal direction. The research establishes the potential of bilayer nickelates as a platform for topological superconductivity, where tunable topological transitions and boundary modes provide a new avenue for probing unconventional pairing.

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.

Using high-resolution angle-resolved photoemission spectroscopy combined with theoretical calculations, this work systematically investigates the single-layer Ruddlesden-Popper nickelate La1.4Sr0.6NiO4 to isolate the intrinsic physics of the NiO layers. The results reveal strong electron correlations, manifested as high-energy kinks in the band dispersion and pronounced orbital-dependent band renormalization; flat Fermi surface sheets are also observed along the diagonal momentum direction, forming a quasi-one-dimensional electronic structure. This square Fermi surface topology gives rise to non-Fermi liquid behavior consistent with the Luttinger liquid model, as evidenced by power-law spectral functions, robust temperature scaling, and spin-charge separation. The study reveals exotic Luttinger liquid behavior emerging from an unexpected dimensional reduction in an intrinsic quasi-two-dimensional nickelate and provides a new perspective for understanding the physics of multilayer nickelates.

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.

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.

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.

This study systematically investigates the structural phase transitions and electronic property evolution of the bilayer nickelate La₃Ni₂O₇ under pressure and temperature using high-pressure high-temperature Raman spectroscopy and synchrotron infrared reflectance spectroscopy. Raman measurements confirm a pressure-driven structural phase transition from the tilted Amam phase to the untilted Fmmm or I4/mmm phase, with the emergence of Fano line shapes indicating enhanced electron-phonon coupling. High-temperature data reveal an upper temperature limit of 544 K for this transition at ambient pressure, refining the temperature-pressure phase diagram. Infrared reflectivity measurements show that the phase transition is accompanied by an increase in carrier density by nearly two orders of magnitude, marking a crossover from a bad metal to a good metal. The experiments establish a unified picture where the structural phase transition is strongly coupled to electronic properties, with superconductivity emerging at approximately 6-7 GPa closely associated with the appearance of the untilted phase; however, high symmetry and metallicity alone are insufficient to induce superconductivity, suggesting that strong electronic correlation effects such as density wave fluctuations may also need to be considered.

The recent discovery of superconductivity in La3Ni2O7–δ with a transition temperature Tc close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to the presence of La4Ni3O10 and other intergrowth phases in La3Ni2O7–δ samples, extracting the intrinsic information from the La3Ni2O7 phase is challenging. In this study, we employed 139La nuclear quadrupole resonance (NQR) measurements to eliminate the influence of other structural phases in the sample and obtain microscopic insights into the DW transition in La3Ni2O7–δ. Below the DW transition temperature TDW ∼ 153 K, we observe a distinct splitting in the ±5/2 ↔ ±7/2 transition of the NQR resonance peak at the La(2) site, while only a line broadening is seen in the ±3/2 ↔ ±5/2 transition peak. Through further analysis of the spectra, we show that the line splitting is due to a unidirectional charge modulation. A magnetic line broadening is also observed below TDW, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state. Our results suggest a simultaneous formation of charge- and spin-density wave orders in La3Ni2O7–δ, thereby offering critical insights into the electronic correlations in Ni-based superconductors.