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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.

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.

Addressing the contradiction in the bilayer nickelate La3Ni2O7 between a bulk nodeless energy gap and d-wave-like features in point-contact transport, the paper theoretically assumes that interlayer pairing driven by strong interlayer antiferromagnetic coupling is the dominant pairing channel and introduces interlayer and intralayer pairing amplitudes in a two-orbital tight-binding model to reproduce the multi-gap structure seen by ARPES/STM. It is pointed out that when point-contact coupling to the two layers is inhomogeneous, a contact potential that is odd under layer-exchange mirror symmetry can act as an effective Zeeman field on the layer pseudospin, thereby binding in-gap Andreev states in the barrier region that shift with barrier width and strength; in the mirror eigenbasis, this barrier mixes mirror sectors with opposite pairing sign and opens a sign-reversal Andreev channel. Simulations for a top-layer barrier on La3Ni2O7 show that its subgap conductance can reproduce the orientation-dependent zero-bias conductance peaks and side peaks usually taken as a d-wave fingerprint, indicating that a nodeless interlayer pairing state can mimic d-wave Andreev signals and providing a natural picture for reconciling conflicting experimental evidence.