Paper

Using polarization-resolved Raman scattering combined with a two-orbital model calculation, this study reveals the multi-orbital character of the density wave in the trilayer nickelate La4Ni3O10. In the phonon sector, the oxygen bond-stretching mode exhibits a pronounced intensity anomaly near the density-wave transition temperature T_DW, while the phonon line shape remains essentially symmetric with negligible changes in frequency shift and linewidth, indicating a moderate electron-phonon coupling and suggesting that the density wave is primarily driven by electronic instability rather than by the lattice. In the electronic Raman scattering, as temperature decreases, the spectral weight of the continuum is significantly depleted below ∼114 meV (∼910 cm⁻¹), and a polarization-dependent sharp peak emerges at this energy; this peak is strongest in the B₂g channel and nearly absent in the B₁g channel. By constructing a two-orbital density-wave state model that simultaneously includes Ni-3d_{x²−y²} and Ni-3d_{z²} orbitals and retains the Raman vertices in the orbital space, calculations confirm that this peak corresponds to the density-wave energy gap 2Δ_DW, whose opening displays incoherent and non-mean-field behavior, and that the spectral response necessarily relies on the mixed contributions of both orbitals and cannot be reduced to any single-orbital projection. These results demonstrate that the density-wave instability in La4Ni3O10 has an intrinsic multi-orbital origin, providing crucial experimental and theoretical insights for understanding its competition or coexistence with superconductivity.

Nature of charge density waves and metal-insulator transition in pressurized La₃Ni₂O₇

This study employs neutron scattering techniques to elucidate the spin order and dynamics in bilayer nickelate La₃Ni₂O₇ single crystals. In the ambient-pressure parent phase, clear spin excitations are observed at the reciprocal-space position Q = (0, 0.5, 2.5), exhibiting a spin gap of approximately 5 meV and pronounced in-plane anisotropic dispersion—band-edge softening along the transverse direction reveals competing exchange interactions. The excitations display an out-of-plane modulation with bilayer periodicity, directly confirming antiferromagnetic interlayer coupling. Based on linear spin-wave theory, the experimental dispersion can be accurately described by a bilayer Heisenberg Hamiltonian incorporating strong interlayer exchange and competing in-plane couplings, with a stripe-type magnetic order. After normalizing the spectral intensity to absolute units, it is found that although the spin-wave bandwidth is only 25% of that in cuprates, the local dynamical magnetic susceptibility is significantly enhanced at comparable energies, and the total fluctuating magnetic moment is comparable to that of cuprates. These results reveal that intermediate-energy spin excitations originating from strong electronic correlations are an intrinsic feature of this system, establishing a magnetic framework fundamentally distinct from that of cuprates and providing direct evidence for understanding the pairing mechanism of superconductivity in this system.

By combining density functional theory, dynamical mean-field theory, and the random phase approximation to solve the superconducting gap equation, researchers have discovered that hole-doped layered nickel oxide La₃₋ₓSrₓNi₂O₇ can achieve bulk superconductivity under ambient pressure. When the doping concentration x approaches 0.4, the γ Fermi pocket derived from the Ni-d₃z²₋r² orbital evolves from a circular to a diamond shape and expands to half the Brillouin zone, forming a nearly perfect Fermi surface nesting with an optimal nesting vector Q=(π, π). This structure significantly enhances antiferromagnetic spin fluctuations, elevating the superconducting eigenvalue to experimentally observable levels without the need for high pressure or strain. This work elucidates the mechanism by which hole doping modulates the shape and size of the Fermi pocket, providing a theoretical foundation and an experimentally feasible pathway for realizing the long-sought bulk superconductivity in such materials under ambient conditions.

This study points out that Zhu et al., when measuring the superconducting volume fraction of pressurized Ruddlesden-Popper nickelate La₄Ni₃O₁₀, employed a previously unreported calculation equation, leading to a significant overestimation of the results. By reanalyzing the original data published by Zhu et al. using standard methods for calculating superconducting magnetic moments, the authors found that the superconducting volume fraction is only 51% to 59%, rather than the 81% to 86% reported by Zhu et al. Upon examining the equation and its derivation provided by Zhu et al., the authors discovered that the equation mistakenly used sample geometry parameters in its calculation, resulting in an approximately two-fold overestimate of the volume proportion occupied by the superconducting phase. Using a hypothetical sample as an example, the authors demonstrate that even if the superconducting phase actually accounts for only 50%, this equation would still yield a result close to 100%. Consequently, this error affects all previously reported superconducting volume fraction data for Ruddlesden-Popper nickelates, necessitating a re-evaluation of these conclusions.

This study addresses the controversy over the superconducting pairing symmetry of the high-pressure bilayer nickelate La₃Ni₂O₇ by employing a full-spectrum model based on orthogonalized projection (the Oroj method), which projects Kohn–Sham states onto local Ni-e_g orbitals and, while preserving the density functional theory band structure, redistributes spectral weight over a wider energy range, thereby incorporating Ni–O hybridization and contributions from electronic states away from the Fermi surface. Compared with a Wannier model that describes only the low-energy bands near the Fermi surface, this full-spectrum model significantly enhances interlayer spin fluctuations and yields a commensurate magnetic instability; within the spin-fluctuation framework, such features favor the formation of a sign-changing s±-wave superconducting state, whereas the low-energy model tends toward d-wave pairing. The results indicate that interlayer coupling and Ni–O hybridization in the full-energy description play an important stabilizing role in theoretical predictions of the superconducting pairing symmetry of bilayer nickelates.

This study systematically characterizes the evolution of unoccupied states in the infinite-layer nickel oxide La₁₋ₓCaₓNiO₂ as a function of doping and temperature using O K-edge and Ni L-edge X-ray absorption spectroscopy. Superconductivity emerges in the doping range of x = 0.18 to 0.27. Near x ≈ 0.20–0.23, a redistribution of low-energy spectral weight occurs: Ni 3d-dominated states decrease while O 2p hybridized states increase, indicating an orbital-selective crossover in Ni–O covalency. This crossover coincides precisely with the sign reversal of the Hall coefficient and precedes the suppression of the superconducting critical temperature at higher doping levels. By directly linking transport anomalies and the superconducting dome to measurable Ni–O orbital reorganization, these results represent a critical step toward establishing a unified orbital-resolved phase diagram for infinite-layer nickelates and offer a practical route for designing superconductivity through hybridization engineering.

This study reports the transport properties of epitaxial bilayer La₃Ni₂O₇ thin films grown on LaAlO₃(001) and SrLaAlO₄(001) substrates, modulated by high-pressure oxygen annealing and hydrostatic pressure. Under ambient pressure, films on LaAlO₃ substrates exhibit Fermi liquid metallic behavior with a slight Kondo-like upturn at low temperatures; upon applying hydrostatic pressure from 0.53 to 1.41 GPa, the temperature dependence of resistance gradually evolves into non-Fermi liquid behavior, approaching approximately ~T¹⁴ at 1.41 GPa. Notably, this pressure is only 6–8% of that required to achieve similar effects in single crystals using diamond anvil cells, revealing unexpectedly strong tunability in the thin-film form. Additionally, signs of spin-density wave (SDW) order are observed in films on YAlO₃ substrates but suppressed on LaAlO₃ substrates. Hall effect measurements indicate a multiband electronic structure. These results demonstrate that La₃Ni₂O₇ thin films can approach a strongly fluctuating ordered state under moderate pressures, offering a new pathway for studying the origin of non-Fermi liquid behavior and high-pressure superconductivity in thin-film systems.

Using ultrafast optical spectroscopy on La₄Ni₃O₁₀ single crystals, researchers observed an abrupt change in quasiparticle relaxation dynamics at the density-wave transition temperature of approximately 136 K, revealing the opening of a strongly coupled energy gap of about 52 meV. Multiple coherent phonon modes, including Ag modes near 3.88, 5.28, and 2.09 THz, exhibited mode-selective anomalies across the transition, with the renormalization behavior of the 3.88 THz phonon in particular shifting from conventional anharmonic decay at high temperatures to pronounced hardening at low temperatures, indicating strong coupling between the density-wave instability and lattice degrees of freedom and suggesting that electron–phonon interactions likely play a critical role. Under high excitation fluence, the density wave is suppressed non-thermally, yielding a temperature–fluence phase diagram that resembles the pressure-tuning behavior, though the gap remains relatively stable, leading to an increased coupling ratio. These findings establish the density wave in La₄Ni₃O₁₀ as a lattice-entangled instability involving multiorbital physics and confirm that ultrafast photoexcitation can serve as a non-equilibrium control parameter to effectively suppress density-wave order in nickelates.

This review provides a comprehensive overview of current research on the structural, electronic, and magnetic characteristics of the recently discovered high-temperature superconductor La3Ni2O7 under high pressures. We present the experimental results for synthesizing and characterizing this material, derived from measurements of transport, thermodynamics, and various spectroscopic techniques, and discuss their physical implications. We also explore theoretical models proposed to describe the electronic structures and superconducting pairing symmetry in La3Ni2O7, highlighting the intricate interplay between electronic correlations and magnetic interactions. Despite these advances, challenges remain in growing high-quality samples free of extrinsic phases and oxygen deficiencies and in developing reliable measurement tools for determining diamagnetism and other physical quantities under high pressures. Further investigations in these areas are essential to deepening our understanding of the physical properties of La3Ni2O7 and unlocking its superconducting pairing mechanism.