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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.

Combining first-principles calculations, a pressure-dependent two-orbital model, self-consistent FLEX calculations, and the linearized Eliashberg equation, this study reveals the microscopic mechanism of pressure-enhanced and robust superconductivity in infinite-layer La₀.₈Sr₀.₂NiO₂. The results show that increasing pressure enlarges the kinetic energy scale, reduces the effective correlation strength U/t, enhances interlayer hybridization, and transfers holes from the La/Sr charge reservoir to the correlated Ni region; at low pressure, the enlarged kinetic energy scale and the approach to optimal intermediate coupling promote pairing, whereas at high pressure, pressure-induced self-doping drives the system into the overdoped regime and suppresses superconductivity, thereby forming a broad superconducting dome. Although compression significantly three-dimensionalizes the Fermi surface, the spin susceptibility relevant to pairing depends weakly on q_z and still peaks mainly near (π,π), so the Ni d_{x²−y²}-dominated d-wave pairing state remains stable over the calculated pressure range. This constrained low-energy pairing framework explains the unusual robustness of superconductivity in this material under megabar-level compression.

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We report a 139La-NMR study of polycrystalline samples of multi(n)-layered nickelates, La3Ni2O7−δ (n = 2) and La4Ni3O10−δ (n = 3), at ambient pressure. Measurements of the nuclear magnetic resonance (NMR) spectra and nuclear spin relaxation rate (1/T1) indicate the emergence of a density wave order with a gap below T* ∼ 150 K for La3Ni2O7−δ and ∼130 K for La4Ni3O10−δ. The finite value of 1/T1 below T* indicates metallic ground states with the remaining density of states at the Fermi level (EF) under the density wave order. These features are attributed to multiple d electron bands with different characteristics. Above T*, the gradual decrease in 1/T1T upon cooling implies the presence of a band with flat dispersion near EF. From our microscopic probes, we point out that these nickelates (n = 2 and 3) possess similar electronic states despite the difference in the formal valence of the Ni d electron states, which provides a basis for understanding the novel high-Tc superconductivity under high pressures.

By combining linear terahertz time-domain spectroscopy with third-harmonic generation, this study systematically probes the superconducting pairing symmetry and normal-state pseudogap in compressively strained (La,Pr)₃Ni₂O₇ thin films. Linear terahertz spectroscopy reveals a significant suppression of low-frequency spectral weight below the superconducting transition temperature, accompanied by a weak coherence peak and a large residual conductivity persisting down to near-zero temperature, consistent with a disordered s±-wave pairing scenario. The nonlinear third-harmonic signal sharply enhances upon entering the superconducting state, but its response persists above the superconducting transition temperature, exhibiting a kink at approximately 100 K, which is attributed to the normal-state pseudogap based on similar temperature scales observed in angle-resolved photoemission spectroscopy on analogous films. This study establishes (La,Pr)₃Ni₂O₇ as a bulk superconductor with s±-wave-like pairing, where superconductivity coexists and likely competes with another ordered state, providing a new platform for exploring unconventional superconducting mechanisms beyond cuprates and iron-based superconductors.

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.

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