Daily Overview: Today’s highlights focus on deepening the understanding of the electronic structure of hybrid Ruddlesden–Popper nickelates. Two systematic muon spin rotation and resistivity studies, investigating the density wave orders in trilayer La₄Ni₃O₁₀ and bilayer La₃Ni₂O₇ respectively, employ pressure tuning and oxygen isotope substitution to reveal the intrinsic intertwining nature of spin density waves and charge density waves, as well as their distinct microscopic origins—the former primarily arising from electronic correlations, while the latter is significantly driven by electron–phonon coupling. The findings further suggest that suppressing the CDW order may be the key to achieving high-pressure superconductivity. These conclusions provide important constraints for understanding the pairing mechanism of high-pressure superconductivity in Ruddlesden–Popper nickelates. Additionally, a theoretical framework article starts from the ionically bonded O-/M bridging carrier pairing, unifying the pairing picture above room temperature in both cuprates and nickelates, and lists 32 pieces of experimental evidence to support its universality. Another study based on ARPES spectral scaling reveals that various strongly correlated materials, including La₃Ni₂O₇, exhibit universal spectral collapse in the marginal dynamics regime, indicating a universal dynamical origin for incoherent electronic states. Although these two works do not entirely focus on nickelates, they provide cross-support for the microscopic picture of their superconducting pairing mechanism and the unified understanding of their spectral features, respectively. arXiv submission processing window: 2026-03-11 00:00 to 2026-03-11 00:00 UTC.

1. Effect of Pressure and Oxygen-Isotope Substitution on Density-Wave Transitions in La$_4$Ni$_3$O$_{10}$

Summary: Through muon spin rotation/relaxation and resistivity measurements combined with oxygen isotope substitution, the pressure and isotope effects on density wave transitions in the trilayer Ruddlesden-Popper nickelate La₄Ni₃O₁₀ were systematically investigated. Under ambient pressure, two incommensurate spin density wave (SDW) transitions were observed at 132 K and 80–90 K; the magnetic structure reveals that the outer two Ni layers exhibit an antiferromagnetically coupled SDW order, while the inner layer has a smaller magnetic moment, and a c-axis component of the magnetic moment emerges below T*. The abrupt onset of the internal field at T_SDW indicates that the SDW transition resembles a first-order phase change and is closely intertwined with the charge density wave (CDW) occurring at the same temperature. Under applied pressure, T_SDW, T*, and T_CDW are uniformly suppressed at a rate of approximately -13 K/GPa, differing from the behavior in bilayer La₃Ni₂O₇ where pressure increases the separation between SDW and CDW. Substitution of ¹⁶O with ¹⁸O raises T_CDW; in the region where CDW and SDW are intertwined, T_SDW also exhibits a significant isotope effect similar in magnitude to the shift in T_CDW, whereas no isotope effect is observed for the SDW at T* where it evolves independently. These results reveal the strongly intertwined nature of SDW and CDW in La₄Ni₃O₁₀ and suggest that pressure-induced suppression of the CDW order may be a key mechanism for high-pressure superconductivity in Ruddlesden-Popper nickelates.

2. Oxygen-isotope effect on density wave transitions in La$_3$Ni$_2$O$_{7}$

Summary: This study systematically explores the isotope effects on the charge density wave (CDW) and spin density wave (SDW) transitions in the bilayer Ruddlesden-Popper nickelate La₃Ni₂O₇ through oxygen isotope substitution (¹⁶O→¹⁸O) using resistivity and muon spin rotation (μSR) experiments. Resistivity measurements reveal a significant increase in the CDW transition temperature by approximately 6 K after ¹⁸O substitution, while μSR results indicate that the SDW transition temperature remains unaffected within experimental error. Raman spectroscopy confirms the effectiveness of the isotope substitution and the softening of lattice phonon modes. This contrasting isotope response suggests that lattice vibrations, i.e., electron-phonon coupling, play a crucial role in the formation of the CDW order, whereas the SDW order primarily originates from electronic interactions. The findings unveil distinct microscopic origins of the two density wave orders and hint at the potential relevance of electron-phonon coupling to the superconducting pairing mechanism in Ruddlesden-Popper nickelates, providing key constraints for theoretical models.

3. Ionic-Bond-Driven Atom-Bridged Room-Temperature Cooper Pairing in Cuprates and Nickelates: a Theoretical Framework Supported by 32 Experimental Evidences

Summary: Addressing the long-standing puzzle of the pairing mechanism for high-temperature superconductivity in cuprates and nickelates, this paper proposes a picture of itinerant Cooper pairs mediated by oxygen-bridged electron pairs (e⁻-O-e⁻) or metal-bridged hole pairs (h⁺-M-h⁺), based on the dominant role of ionic bonds on the order of eV, the electron affinities of O⁻ and O²⁻ (1.46 eV and -8.08 eV, respectively), and the large double ionization energies of metal atoms (approximately 15–28 eV). Such pairing forms below the pseudogap temperature T*, which is higher than Tc, and follows the relationship of chemical bond → structure → properties, being applicable to cuprates, nickelates, iron-based, and other ionic superconductors. The author verifies the correctness and universality of this mechanism through 32 independent experimental pieces of evidence, especially STM images within the CuO₂ plane and the extremely small pairing size, and points out that any sub-eV or covalent bonding pairing mechanism is unreliable. This theory reveals the missing link between ionic bonds and superconductivity, resolves a four-decade-long puzzle, and demonstrates the feasibility of achieving room-temperature carrier pairing in ionic-bond superconductors. Based on this, the author establishes a new theoretical framework centered on the strongest pairing strength and Bose–Einstein condensation, opening a new path for understanding the mechanism of high-temperature superconductivity and bringing the dream of room-temperature superconductivity closer to reality.

4. Evidence of universal spectral collapse at a marginal dynamical regime

Summary: This study proposes that incoherent electronic states in strongly correlated materials arise not from disorder or material-specific mechanisms, but from self-generated dynamical disorder induced by competing fluctuations. In this marginal dynamical regime, electron dynamics naturally couple with time-dependent scattering, yielding the spectral function form ρ(z)=exp(-z²/4)D_ν(z), where z is the scaled energy, D_ν is the parabolic cylinder function, and ν=-1/2 is fixed. By independently scaling the angle-resolved photoemission spectroscopy (ARPES) energy distribution curves of the cuprates Nd₂₋ₓCeₓCuO₄ and Bi₂Sr₂CaCu₂O₈₊δ, the Kagome metal CsCr₃Sb₅, and the bilayer nickelate La₃Ni₂O₇, all datasets collapse onto a single universal curve, with only the amplitude and energy scale varying among materials. This spectral collapse indicates that microscopic details such as lattice geometry, band structure, and chemical composition become irrelevant in the low-energy regime, exhibiting fixed-point-like dynamical behavior. The result establishes a unified quantitative framework for the continuously dominant ARPES spectra across diverse strongly correlated materials.