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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.

Using resonant inelastic X-ray scattering (RIXS) at the oxygen K-edge, plasmon collective excitations were observed in the low-valence nickelate Pr4Ni3O8 and compared with the overdoped cuprate La₂₋ₓSrₓCuO₄. The experiments revealed that the nickelate plasmons exhibit dispersive behavior within the in-plane momentum, but with significantly lower velocity and stronger damping than the cuprate, and they become overdamped and disappear at much smaller momenta. Random phase approximation (RPA) calculations indicate that these differences originate from reduced electron hopping and enhanced long-range Coulomb interaction screening in the nickelate, where both the in-plane hopping integral and Coulomb interaction strength are substantially smaller than in the cuprate. Furthermore, the out-of-plane plasmons in the nickelate showed no discernible dispersion, possibly due to its trilayer coupling structure. Temperature-dependent studies found that the plasmons in Pr4Ni3O8 soften with increasing temperature, whereas in the cuprate the energy remains nearly constant while damping increases, suggesting the presence of additional correlations such as stripe fluctuations in the nickelate. These results reveal a unique charge screening landscape in nickelates, where weakened electron hopping and enhanced Coulomb screening are key distinguishing features from cuprates, potentially explaining the lower superconducting transition temperature of nickelates and providing quantitative experimental constraints for analogies between the two material families.

Using ultra-low-temperature scanning tunneling microscopy/spectroscopy and electrical transport measurements, this study reports for the first time the observation of an energetically symmetric, flat-bottomed U-shaped superconducting gap with zero residual density of states and a gap magnitude exceeding 40 meV in (La,Pr)₃Ni₂O₇ thin films. Transport measurements reveal an onset superconducting transition temperature above 40 K and a zero-resistance temperature above 20 K under ambient pressure. The tunneling spectra exhibit unconventional temperature evolution: as temperature increases, the U-shaped gap rapidly fills and transforms into a V-shaped gap; meanwhile, the gap decreases under a 14-tesla c-axis magnetic field. These temperature and magnetic field dependencies are consistent with superconducting gap behavior, indicating the existence of a nodeless gap function at ultralow temperatures. This discovery unveils the nature of high-temperature superconductivity in bilayer nickelates and provides important insights into achieving superconductivity above the boiling point of liquid nitrogen under ambient or zero pressure.

This response paper clarifies the evaluation method of the superconducting shielding volume fraction in pressurized Ruddlesden-Popper nickelates. The authors point out that their method directly follows the standard magnetostatic self-consistency relation for finite samples (Equations 2–4), where the measured susceptibility is corrected by the demagnetization factor N to obtain the intrinsic susceptibility, from which the shielding volume fraction is estimated as f ≈ −χ. This method has been widely adopted in the superconductor literature for decades. Taking single-crystal sample S6 as an example, the self-consistent formula yields a superconducting shielding volume fraction of approximately 86% at 50 GPa and 5 K, and about 82% at 40 GPa. The authors argue that the fundamental flaw in the critique presented in arXiv:2602.19282 lies in the assumption by the opposing party that the measured diamagnetic moment is linearly proportional to the superconducting shielding volume fraction, and their simple normalization via calculating the full-shielding Meissner moment. This assumption is invalid for thin disk-shaped samples with strong demagnetization, because the internal field and magnetization are self-consistently coupled through the demagnetizing field, resulting in a nonlinear relationship between the measured moment and the shielding fraction, and consequently yielding an underestimated value of about 60% by the opposing method. The paper also discusses the applicability of the single-demagnetization-factor framework, noting that the sample has a uniform structure and high quality, supporting this macroscopic description, while the artificially constructed phase-separation model by the opposing party is not applicable here. The authors conclude that the method based on magnetostatic self-consistency is correct and a widely adopted standard approach.

Zhang et al. [1] reported zero-field cooling (ZFC) and field cooling (FC) data for Pr₄Ni₃O₁₀ single crystals under high pressure, confirming bulk superconductivity in Ruddlesden-Popper nickelates and estimating a superconducting volume fraction f = 0.85 using the amplitude of the internal susceptibility χ_internal. This paper questions the key assumption of that method, namely f = |χ_internal|, and demonstrates through a counterexample that even when the calculated |χ_internal| for a sample reaches 0.82, the actual superconducting volume fraction f may be below 0.10. The authors point out that the calculation of the demagnetizing factor N depends on the distribution of non-superconducting regions within the sample, and the homogeneity assumption does not hold—if 90.2% of the volume is non-superconducting phase, the measured susceptibility can still match the original results as long as the geometry alters the demagnetizing factor. Therefore, χ_internal alone cannot uniquely determine f, and this approach requires reexamination across the entire field of superconductivity research.

This study investigates the newly discovered nickel-based superconductor La₃Ni₂O₇ and reveals the “orbital dimerization” mechanism underlying its first-order structural phase transition. The authors note that standard density functional theory (DFT) and the LDA+DMFT method incorporating intra-atomic correlations fail to reproduce the abruptness of this transition because they neglect crucial inter-atomic correlations. By constructing a many-body effective Hamiltonian that includes low-energy active orbitals and performing exact diagonalization, they find that when the Ni-O-Ni bond angle approaches linearity, strong antiferromagnetic superexchange interactions emerge between interlayer Ni orbitals, forming spin-singlet bonds that cause a sharp drop in total energy. This energy reduction creates a new local energy minimum at a specific configuration that coexists with the tilted configuration, perfectly explaining the first-order transition characteristics observed in experiments as well as the coexistence of high- and low-pressure structures. This mechanism not only alters the lattice bonding properties but also leads to qualitative changes in the low-energy electronic structure, such as the emergence of superconductivity. The computational framework of DFT plus many-body corrections established in this work is universal and applicable to ionic materials containing open-shell d/f electrons, providing key microscopic insights into understanding the structure–electronic property relationships in such systems.

The report of superconductivity (SC) with Tc~80 K in bilayer Ruddlesden-Popper (RP) nickelate La3Ni2O7-delta have sparked considerable investigations on its normal state properties and SC mechanism under pressure and at low temperature. It is believed that the density wave (DW) at ~150 K plays an important role in SC emergence, but its nature remains largely underexplored. Here, we utilized temperature-dependent in-situ Ni K-edge X-ray Absorption Near-edge Spectroscopy (XANES) to probe the Ni-3d/4p electronic states of La3Ni2O7-delta and La2PrNi2O7-delta samples down to 4.8 K, enabling us to witness the evolution of both in-plane d_(x^2-y^2)/p_x (p_y) and out-of-plane d_(3z^2-r^2)/p_z orbitals of NiO6 octahedron across the DW transition. Main edge energy associated with Ni 4p orbital shows an anomalous decline near DW transition, signifying the occurrence of lattice distortions as a hallmark of charge density wave. Below DW transition, the enlarged crystal field splitting (CFS) indicates an enhanced NiO6 octahedral distortion. Intriguingly, magnetic Pr substituents could activate the mutual interplay of d_(x^2-y^2) and d_(3z^2-r^2) orbitals. We discussed its relevance to the favored bulk SC in the pressurized polycrystalline La2PrNi2O7-delta than pristine.

This study employs the variational Monte Carlo method to investigate superconductivity in a two-band t-J model consisting of an itinerant orbital (orbital 0) and a quasi-localized orbital (orbital 1). The key finding is the emergence of a robust orbital-selective d-wave superconducting state, which originates entirely from the itinerant orbital 0. Analysis of the superexchange energy hierarchy reveals that the quasi-localized orbital 1 competes with superconductivity by favoring the formation of local inter-orbital bound states, which act as energy defects that disrupt phase coherence. Consequently, the superconducting order parameter decreases monotonically with increasing occupation of orbital 1. Inspired by superconductivity in the nickelate La₃Ni₂O₇, these results highlight the crucial role of multiorbital physics beyond the single-band t-J framework and identify a concrete pathway for enhancing the superconducting transition temperature: suppressing the involvement of the localized d_{z²} orbital.

By constructing the global phase diagram of a bilayer two-orbital Hubbard model, we investigate the orbital-selective electron correlations in the bilayer nickelate La3Ni2O7. At half-filling the system undergoes a Mott transition, and at the physical electron filling it exhibits strong orbital selectivity, manifested as interlayer spin singlet formation among z²-orbital electrons. This effect leads to significant band-structure renormalization with dramatic bandwidth narrowing, while the splitting between the z² bonding and antibonding bands remains at about 1 eV. These correlation features naturally explain the orbital-dependent effective mass enhancement and the sinking of the z² bonding band below the Fermi level observed in ARPES experiments, as well as the puzzling contrast in optical conductivity where the Drude weight drops sharply yet the interband peak position barely shifts. Theoretical analysis reveals that interlayer antiferromagnetic superexchange plays a key role in maintaining the band splitting, and intralayer correlations drive the z² band away from the Fermi surface. This work provides a unified microscopic picture for understanding the normal-state properties and the mechanism of high-temperature superconductivity in multilayer nickelates.