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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.

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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.

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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.

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