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This study systematically characterizes the evolution of unoccupied states in the infinite-layer nickel oxide La₁₋ₓCaₓNiO₂ as a function of doping and temperature using O K-edge and Ni L-edge X-ray absorption spectroscopy. Superconductivity emerges in the doping range of x = 0.18 to 0.27. Near x ≈ 0.20–0.23, a redistribution of low-energy spectral weight occurs: Ni 3d-dominated states decrease while O 2p hybridized states increase, indicating an orbital-selective crossover in Ni–O covalency. This crossover coincides precisely with the sign reversal of the Hall coefficient and precedes the suppression of the superconducting critical temperature at higher doping levels. By directly linking transport anomalies and the superconducting dome to measurable Ni–O orbital reorganization, these results represent a critical step toward establishing a unified orbital-resolved phase diagram for infinite-layer nickelates and offer a practical route for designing superconductivity through hybridization engineering.

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This study reports the transport properties of epitaxial bilayer La₃Ni₂O₇ thin films grown on LaAlO₃(001) and SrLaAlO₄(001) substrates, modulated by high-pressure oxygen annealing and hydrostatic pressure. Under ambient pressure, films on LaAlO₃ substrates exhibit Fermi liquid metallic behavior with a slight Kondo-like upturn at low temperatures; upon applying hydrostatic pressure from 0.53 to 1.41 GPa, the temperature dependence of resistance gradually evolves into non-Fermi liquid behavior, approaching approximately ~T¹⁴ at 1.41 GPa. Notably, this pressure is only 6–8% of that required to achieve similar effects in single crystals using diamond anvil cells, revealing unexpectedly strong tunability in the thin-film form. Additionally, signs of spin-density wave (SDW) order are observed in films on YAlO₃ substrates but suppressed on LaAlO₃ substrates. Hall effect measurements indicate a multiband electronic structure. These results demonstrate that La₃Ni₂O₇ thin films can approach a strongly fluctuating ordered state under moderate pressures, offering a new pathway for studying the origin of non-Fermi liquid behavior and high-pressure superconductivity in thin-film systems.

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.