The discovery of pressure-induced high temperature superconductivity in the bilayer nickelate La_3Ni_2O_7 has raised the question of how its spin-density-wave (SDW) state evolves toward the superconducting regime. Here, we report a systematic electronic Raman study of La_3Ni_2O_7 single crystals under hydrostatic pressures up to 16.51 GPa. Both the SDW gap energy and the transition temperature T_SDW show an overall increase with pressure, while the dimensionless coupling ratio 2Δ_SDW/(k_BT_SDW) remains constant around ∼7.5, indicating a robust strong-coupling character of SDW state. At the same time, the Raman SDW peak broadens as pressure is applied, indicating a gradual weakening of long-range SDW order. These results reveal an unusual pressure evolution in which the SDW energy scale is enhanced while the SDW state becomes progressively less coherent, providing spectroscopic constraints on the magnetic correlations relevant to superconductivity in bilayer nickelates.

Motivated by the discovery of superconductivity in bilayer La3Ni2O7 at 80 K and the increased superconducting transition temperature, Tc, up to 96 K in single crystals of La1.57Sm1.43Ni2O7-delta under pressure, we systematically study the effect of Sm substitution on the superconductivity and structure of La3-xSmxNi2O7 (0 x 1.5) under pressure. Experimental investigations in polycrystalline samples reveal that Sm substitution monotonically decreases the lattice constants c and a, thereby enhancing crystal structural distortion and leading to an evolution of the metallic ground state in La3Ni2O7 to an insulating state in La1.5Sm1.5Ni2O7. The optimal Tonset c rises with increasing Sm substitution, and the maximum Tonset c is 89.2 K in polycrystalline La1.5Sm1.5Ni2O7. Divergences in Tc compared with single crystals of La1.57Sm1.43Ni2O7-delta are likely due to variations in the microstructure induced by different material growth approaches. Our results suggest that the enhancement of Tc in La3-xSmxNi2O7 is mainly affected by the compressed c lattice before saturation, and the superconducting transition pressure increases with substitution concentration. Our experimental results provide insight into the influence of small-radius rare-earth element substitution on nickelate superconductors, offering a means to further increase the transition temperature.

The discovery of superconductivity in Ruddlesden-Popper nickelates has established a new frontier in the study of high-temperature superconductors. However, the underlying pairing mechanism and its relationship to the material's electronic and magnetic ground states remain elusive. Since unconventional superconductivity often emerges from a complex interplay of magnetic correlations, elucidating the magnetic ground state of the nickelates at ambient pressure is crucial for understanding the emergence of superconductivity under high pressure. Here, we combine high-resolution angle-resolved photoemission spectroscopy with tight-binding model simulation to investigate the electronic structure of the representative trilayer Ruddlesden-Popper nickelate La_4Ni_3O_10. We provide the first experimental evidence of band splitting induced by interlayer coupling and further resolve the momentum-dependent density wave gap structures along all the Fermi surfaces. Our findings identify the mirror-selective Fermi surface nesting as the origin of the interlayer antiferromagnetic spin density wave and demonstrate the dominant role of Ni-3d_z^2 orbitals in the low-energy physics of La_4Ni_3O_10. These results provide a fundamental framework for understanding the magnetic interactions and high-temperature superconductivity mechanism in the Ruddlesden-Popper nickelate family.

High-T_{c} superconductivity has recently been discovered in Ruddlesden-Popper (RP)-phase nickelates under pressure, where the low-energy electronic structure is dominated by Ni d_{x^{2}−y^{2}} and d_{z^{2}} orbitals. However, the respective roles of these orbitals in superconductivity remain unclear. Here, by combining x-ray absorption, electron energy loss spectroscopy, and density functional theory calculations on La_{4}Ni_{3}O_{10} single crystals, we identify ligand holes in the p_{x,y} orbitals of planar oxygen and the p_{z} orbitals of apical oxygen, which hybridize with the Ni d_{x^{2}−y^{2}} and d_{z^{2}} orbitals, respectively. These ligand holes enable orbital-selective O K-edge resonant inelastic x-ray scattering (RIXS) study, which reveals that d_{x^{2}−y^{2}} states dominate the low-energy charge excitations and are more itinerant. We also observe an ∼0.1eV bimagnon through RIXS and Raman spectroscopy. Our results reveal distinct contributions of Ni d_{x^{2}−y^{2}} and d_{z^{2}} orbitals to the electronic and magnetic structure and provide direct experimental insights to understand the RP-phase nickelate superconductors.

We report a polarized ultrafast pump-probe study of the normal-state electronic dynamics in bilayer La_3Ni_2O_7 and trilayer La_4Ni_3O_10 single crystals at ambient pressure. While both nickelates exhibit density-wave (DW) transitions accompanied by the opening of a quasiparticle relaxation bottleneck, their electronic responses display strikingly different symmetry properties. La_4Ni_3O_10 maintains an isotropic optical response across the entire temperature range. In contrast, La_3Ni_2O_7 exhibits a pronounced twofold (C_2) anisotropy in its low-temperature electronic dynamics. This electronic nematicity, evident in both the relaxation dynamics and the effective gap scales, competes with a secondary isotropic order emerging below 115 K. The presence of macroscopic electronic anisotropy in the bilayer system, and its absence in the trilayer system, suggests an intimate relation between electronic nematic fluctuations and superconducting pairing in La_3Ni_2O_7 that worth for deeper explorations.

The synthesis of high-quality Ruddlesden-Popper (RP) nickelates remains challenging due to variations in oxygen content and the prevalence of intergrown RP phases. Precisely controlling the stoichiometry and characterizing the resulting physical properties are essential for understanding the mechanism of high-T_c superconductivity in these materials. In this work, we synthesize a series of La_3Ni_2O_7+δ samples with systematically controlled oxygen content and perform comprehensive structural and compositional analyses. Precise oxygen tuning enables us to tailor the microstructure, yielding a pure bilayer phase, a mixture of bilayer and hybrid single-layer-bilayer phases, and a predominantly bilayer phase containing trilayer intergrowths. High-pressure transport measurements reveal distinct superconducting transitions with contrasting T_c values, corresponding to the bilayer phase, the hybrid phase, and trilayer inclusions. Notably, we find that oxygen content not only governs the phase purity-i.e., the presence of intergrowth phases-but also directly modulates the upper critical field (H_c2) of the bilayer superconductivity. By establishing a phase diagram of T_c and H_c2 as functions of oxygen content in La_3Ni_2O_7+δ, this work advances synthetic control and provides new insights into the superconducting mechanism of RP nickelates.

The bilayer nickelate superconductor La3Ni2O7 undergoes a density wave transition near 150 K that has attracted intensive scrutiny, yet its microscopic origin remains elusive. Here we report polarization-resolved electronic Raman scattering measurements on high-quality single crystals of La3Ni2O7. Below 150 K, we observe a pronounced, symmetry-dependent redistribution of spectral weight in B1g and B2g channels, consistent with the formation of spin-density-wave (SDW) gaps. Quantitative analysis reveals momentum-selective SDW gap amplitudes, with intermediate-to-strong coupling near X/Y points of the Brillouin zone and weaker coupling along the diagonal direction, indicating an unconventional SDW driven by anisotropic electronic correlations. Our results establish the electronic character of the SDW in La3Ni2O7, and provide a microscopic foundation for understanding the emergence of high-temperature superconductivity under pressure in nickelates.

Recently, high-temperature superconductivity has been established in bilayer La3Ni2O7, which exhibits a density-wave (DW) transition at 150 K under ambient pressure. The DW order is believed to be linked to superconductivity, as it is suppressed upon the emergence of superconductivity at high pressures. Here, we explore the ultrafast dynamics of high-energy electronic excitations from 10 K to room temperature under ambient pressure using time-resolved optical spectroscopy. Two high-energy electronic excitations at 1.8 and 2.4 eV, arising from distinct interband transitions, are identified. They exhibit different DW gaps of approximately 54 and 67 meV, respectively, along with relaxation dynamics that can be well described by the Rothwarf-Taylor model. In addition, we observe four coherent Raman-active phonon modes that exhibit distinct coupling with different electronic excitations. The phonon softening with increasing temperature can be well described between 100 K and room temperature by a semi-quantitative model, which includes thermal expansion and anharmonic phonon-phonon coupling. At cryogenic temperatures, deviations from the measured temperature-dependent phonon frequencies and the model fits suggest an additional contribution from electron-phonon coupling. Our study provides direct evidence of the complex gap structure and phonon dynamics in this material, offering critical insights into the DW mechanism and many-body effects.

Systematically controlling the superconducting transition temperature (Tc) in the bilayer Ruddlesden-Popper nickelate La3Ni2O7 remains a significant challenge. Here, we address this by synthesizing high-quality polycrystalline La3−xNdxNi2O7 (0 ≤ x ≤ 2.4) with record-level rare-earth substitution. Nd doping compresses the lattice and enhances the spin density wave (SDW) transition temperature, and elevates the pressure required for the orthorhombic-to-tetragonal structural transition. Superconductivity is observed across all doping levels in high-pressure electronic transport measurements, with the onset Tc rising to ~ 93 K and the resistance derivative indicating the signature of superconductivity reaching 96–97 K for x = 2.1 and 2.4. Using the radio-frequency transmission technique, recently applied to nickelate superconductors, we detect signatures of superconductivity at 100.5 K in the x = 2.1 compound, pushing the Tc frontier further. Our work reveals the critical role of magnetism and provides a structural descriptor for elevating Tc in Ruddlesden-Popper nickelates. The authors synthesize polycrystalline La3-xNdxNi2O7 with record-level rare-earth Nd substitution (x = 2.4) creating chemical pressure. Radio-frequency transmission reveals superconducting signatures up to 100.5 K under 33 GPa pressure.

The discovery of superconductivity in La3Ni2O7-δ under high pressure,with an onset critical temperature around 80 K, has sparked significant interest in the superconducting phases of Ruddlesden-Popper nickelates, Lan+1NinO3n+1. While La4Ni3O10 exhibits nearly 100 superconductivity with Tc 30 K under high pressure, magnetic susceptibility studies on La3Ni2O7-δ, however, reveal a more complex picture, indicating either filamentary superconductivity or that approximately 50 crystal phase becomes superconducting in polycrystalline samples. In this study, we employed scattering-type scanning near-field optical microscopy to visualize nanoscale structural phase separation in La3Ni2O7-δ, identifying enhanced optical conductivity with stripes approximately 183 nm wide. These stripes run diagonally with respect to the Ni-O-Ni bond directions in the a-b plane, ruling out the possibility that they arise from impurity phases, like the '1313', '214' or '4310' structures. The dark regions and bright stripes exhibit optical conductivities 22 respectively. Additionally, we find that the bright stripes constitute about 38 and the transitional region between dark regions and bright stripes. Our results suggest that optical conductivity stripes originate from nanoscale structural phase separation. In contrast, La4Ni3O10 exhibits uniform and higher optical conductivity with no observable evidence of phase separation. Thus, our study represents a pioneering effort to directly image nanoscale phase separation in Lan+1NinO3n+1 nickelates. This observation could provide crucial insights into the factors that limit the superconducting volume fraction of La3Ni2O7-δ, highlighting SNOM as a powerful probe for exploring nanoscale low-energy physics in correlated quantum materials.

The precise crystal structure of La3Ni2O7 in its high-pressure superconducting state has been a subject of intense debate, with proposed models including both orthorhombic and tetragonal symmetries. Using high-pressure Raman spectroscopy combined with frst-principles calculations, we unravel the structural evolution of La3Ni2O7 under pressure up to 32.7 GPa. We identify a clear structural transition sequence: from the orthorhombic Amam phase to a mixed Amam+Fmmm phase at 4 GPa, followed by a complete transition to the tetragonal I4/mmm phase at 14.5 GPa, which is signaled by a pronounced phonon renormalization. The emergence of bulk superconductivity is found to coincide precisely with this transition to the I4/mmm phase. Our results de nitively establish the tetragonal I4/mmm structure as the host of superconductivity in La3Ni2O7, resolving a central controversy and providing a critical foundation for understanding the superconducting mechanism in nickelates.

Motivated by the discovery of superconductivity in bilayer La_3Ni_2O_7 at 80 K and the increased superconducting transition temperature, T_c, up to 92 K in single crystals of La_2SmNi_2O_7 under pressure, we systematically study the effect of Sm doping on the superconductivity and structure of La_3-xSm_xNi_2O_7 (0 ≤ x ≤ 1.5) under pressure. Experimental investigations in polycrystalline samples reveal that Sm doping monotonically decreases the lattice constants c and a, thereby enhancing crystal structure distortion and leading to an evolution of the metallic ground state in La_3Ni_2O_7 to an insulating state in La_1.5Sm_1.5Ni_2O_7. The maximum onset T_c in compounds x=0.9 and 1.5 is 89 K, while the pressure that drives the emergence of superconductivity is higher for higher doping levels. The results suggest that the enhancement of T_c in La_3-xSm_xNi_2O_7 is mainly affected by the compressed c lattice before saturation, and the structure transition is critical for the emergence of superconductivity. Our experimental results provide insight into the influence of elemental substitution on nickelate superconductors, offering a means to increase the transition temperature further.

Systematically controlling the superconducting transition temperature (T_c) in the bilayer Ruddlesden-Popper nickelate La_3Ni_2O_7 remains a significant challenge. Here, we address this by synthesizing high-quality polycrystalline La_3-xNd_xNi_2O_7 (0 ≤ x ≤ 2.4) with record-level rare-earth substitution. Nd doping compresses the lattice, particularly along the c axis, enhances the spin density wave transition temperature, and elevates the pressure required for the orthorhombic-to-tetragonal structural transition. Superconductivity is observed across all doping levels under high pressures, with the onset T_c rising to ∼93 K for x = 2.1 and 2.4 from the electronic transport measurement. Using the radio-frequency transmission technique, newly applied to nickelate superconductors, we detect signatures of superconductivity at 98 ± 2 K in the x=2.4 compound, pushing the T_c frontier further. We identify a universal linear relationship where T_c decreases with the c-axis lattice parameter at a rate of approximately -28 K/Å, demonstrating that enhanced interlayer magnetic exchange coupling is the dominant mechanism for superconducting pairing. Our work establishes the critical role of magnetism and provides a unified structural descriptor for elevating T_c in bilayer nickelates.

The discovery of unconventional superconductivity around 80 K in perovskite nickelates under high pressure has furnished a new platform to explore high-temperature unconventional superconductivity in addition to cuprates. Understanding the normal state of nickelate superconductors is crucial to uncovering the origin of this unconventional superconductivity and gaining further insight into its underlying mechanism. In this study, we systemically studied the transport properties of La3Ni2O7 by tuning the pressure under high magnetic fields. Magnetoresistance (MR) consistently exhibits a quasi-quadratic dependence on the magnetic field across all measured pressures and temperatures. Increased pressure enhances the metallicity of the system and leads to a monotonic increase in MR, which follows the extended Kohler's rule. These results suggest that the normal state of La3Ni2O7 to be a multiband metallic nature.

Identification of superconductivity in the Ruddlesden-Popper phases of nickelates under high pressure remains challenging. Here, we report a comprehensive study of the crystal structure, electrical resistance, and Meissner effect in single crystals of bilayer nickelate La3Ni2O7 under hydrostatic pressures up to 104 GPa. Using high-pressure X-ray diffraction, we observe a structural transition from an orthorhombic to a tetragonal phase above 40 GPa. Superconductivity emerges with a maximum onset transition temperature Tconset of 83 K at 18.0 GPa, accompanied by zero resistance. The superconducting phase is gradually suppressed and vanishes above 80 GPa, forming a right-triangle-like superconducting region. Direct-current magnetic susceptibility measurements demonstrate the Meissner effect and reveal a superconducting volume fraction of ∼41% at 22.0 GPa and 20 K, confirming the bulk nature of superconductivity in La3Ni2O7. Our results highlight the intricate relationship between superconductivity, oxygen content, and structural transitions in this material.

The discovery of high-temperature superconductivity in La3Ni2O7 and La4Ni3O10 under pressure has garnered extensive attention. Herein, we report systematic investigations on the structure, magnetism, and electrical resistance evolutions of Pr4Ni3O10-delta polycrystalline samples under various pressures. Pr4Ni3O10-delta exhibits density wave transitions on Ni and Pr sublattices at approximately 157.6 and 4.3 K, respectively. The density wave can be progressively suppressed by pressure. A structural transformation from the monoclinic P21/a space group to the tetragonal I4/mmm occurs around 20 GPa. An apparent drop in resistance with an evident magnetic field dependence is observed at pressures above 20 GPa, indicating the emergence of superconductivity. The discovery of superconductivity in Pr4Ni3O10-delta broadens the family of nickelate superconductors. Pr4Ni3O10-delta provides a new platform for investigation of the mechanisms of superconductivity in the Ruddlesden-Popper phases of nickelates.

Recently, a signature of high-temperature superconductivity above the liquid nitrogen temperature (77 K) was reported for La3Ni2O7-δ under pressure. This finding immediately stimulated intense interest in the possible mechanism of high-Tc superconductivity in double-layer nickelates. Notably, the pressure-dependent phase diagram inferred from transport measurements indicates that the superconductivity under high pressure emerges from the suppression of density-wave-like order at ambient pressure, which is similar to high-temperature superconductors. Therefore, clarifying the exact nature of the density-wave-like transition is important for determining the superconducting mechanism in double-layer nickelates. Here, nuclear magnetic resonance (NMR) spectroscopy of 139La nuclei was performed to study the density-wave-like transition in a single crystal of La3Ni2O7-δ. At high temperatures, two sets of sharp 139La NMR peaks are clearly distinguishable from a broad background signals, which are ascribed to La(1) sites from two bilayer Ruddlesden-Popper phases with different oxygen vacancy δ. As the temperature decreases, the temperature-dependent 139La NMR spectra and nuclear spin-lattice relaxation rate (1/T1) for both La(1) sites provide evidence of spin-density-wave (SDW) ordering below the transition temperature (TSDW), which is approximately 150 K. The anisotropic splitting in the NMR spectra suggests the formation of a possible double spin stripe with magnetic moments aligned along the c-axis. Furthermore, we studied the pressure-dependent SDW transition up to ∼ 2.7 GPa. Surprisingly, the TSDW inferred from NMR measurements of both La(1) sites increases with increasing pressure, which is opposite to the results from previous transport measurements under pressure and suggests an intriguing phase diagram between superconductivity and SDW. In contrast, the present 139La NMR is insensitive to the possible charge-density-wave (CDW) order in the Ni-O planes. All these results will be helpful for building a connection between superconductivity and magnetic interactions in double-layer nickelates.

Recent discoveries of superconductivity in Ruddlesden-Popper nickelates realize a rare category of superconductors. However, the use of high-pressure diamond anvil cells limits spectroscopic characterization of the density waves and superconducting gaps. Here, we systematically studied the pressure evolution of La_4Ni_3O_10 using ultrafast optical pump-probe spectroscopy. We found that the transition temperature and energy gap of density waves are suppressed with increasing pressure and disappear suddenly near 17 GPa where structural transition appears. In addition, the observation of a single density wave gap indicates that the spin density wave and charge density wave remain coupled as pressure increases, rather than decoupling. After the density wave collapse, a distinct low-temperature regime emerges, characterized by a small gap consistent with potential superconducting pairing. The separated phase region of superconductivity and density waves suggests that superconductivity in pressurized-La_4Ni_3O_10 competes strongly with density waves, offering new insights into the interplay between these two phenomena.