Paper

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.

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.

Nature of charge density waves and metal-insulator transition in pressurized La₃Ni₂O₇

This study employs neutron scattering techniques to elucidate the spin order and dynamics in bilayer nickelate La₃Ni₂O₇ single crystals. In the ambient-pressure parent phase, clear spin excitations are observed at the reciprocal-space position Q = (0, 0.5, 2.5), exhibiting a spin gap of approximately 5 meV and pronounced in-plane anisotropic dispersion—band-edge softening along the transverse direction reveals competing exchange interactions. The excitations display an out-of-plane modulation with bilayer periodicity, directly confirming antiferromagnetic interlayer coupling. Based on linear spin-wave theory, the experimental dispersion can be accurately described by a bilayer Heisenberg Hamiltonian incorporating strong interlayer exchange and competing in-plane couplings, with a stripe-type magnetic order. After normalizing the spectral intensity to absolute units, it is found that although the spin-wave bandwidth is only 25% of that in cuprates, the local dynamical magnetic susceptibility is significantly enhanced at comparable energies, and the total fluctuating magnetic moment is comparable to that of cuprates. These results reveal that intermediate-energy spin excitations originating from strong electronic correlations are an intrinsic feature of this system, establishing a magnetic framework fundamentally distinct from that of cuprates and providing direct evidence for understanding the pairing mechanism of superconductivity in this system.

By combining density functional theory, dynamical mean-field theory, and the random phase approximation to solve the superconducting gap equation, researchers have discovered that hole-doped layered nickel oxide La₃₋ₓSrₓNi₂O₇ can achieve bulk superconductivity under ambient pressure. When the doping concentration x approaches 0.4, the γ Fermi pocket derived from the Ni-d₃z²₋r² orbital evolves from a circular to a diamond shape and expands to half the Brillouin zone, forming a nearly perfect Fermi surface nesting with an optimal nesting vector Q=(π, π). This structure significantly enhances antiferromagnetic spin fluctuations, elevating the superconducting eigenvalue to experimentally observable levels without the need for high pressure or strain. This work elucidates the mechanism by which hole doping modulates the shape and size of the Fermi pocket, providing a theoretical foundation and an experimentally feasible pathway for realizing the long-sought bulk superconductivity in such materials under ambient conditions.

This study points out that Zhu et al., when measuring the superconducting volume fraction of pressurized Ruddlesden-Popper nickelate La₄Ni₃O₁₀, employed a previously unreported calculation equation, leading to a significant overestimation of the results. By reanalyzing the original data published by Zhu et al. using standard methods for calculating superconducting magnetic moments, the authors found that the superconducting volume fraction is only 51% to 59%, rather than the 81% to 86% reported by Zhu et al. Upon examining the equation and its derivation provided by Zhu et al., the authors discovered that the equation mistakenly used sample geometry parameters in its calculation, resulting in an approximately two-fold overestimate of the volume proportion occupied by the superconducting phase. Using a hypothetical sample as an example, the authors demonstrate that even if the superconducting phase actually accounts for only 50%, this equation would still yield a result close to 100%. Consequently, this error affects all previously reported superconducting volume fraction data for Ruddlesden-Popper nickelates, necessitating a re-evaluation of these conclusions.

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.

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.