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This study systematically analyzes the Raman response of superconducting multi-orbital systems using electronic Raman scattering methods, with nickelates as the application target. For three models—a single-layer and a bilayer two-orbital model involving d_{x^2-y^2} and d_{z^2} orbitals, and a bilayer single-orbital model with only d_{x^2-y^2} orbitals—multiple pairing symmetries including d-wave, s±-wave, and s-wave are considered, and the response characteristics under various Raman symmetries (A1g, B1g, B2g) are calculated. In the two-orbital models, a full multi-orbital approach is employed, incorporating both intra-orbital and inter-orbital scattering, and compared with the additive approximation that simply sums the Raman responses of individual bands. The results reveal distinct fingerprint features in the Raman spectra for different pairing symmetries and model structures, with the full multi-orbital calculations uncovering inter-orbital mixing effects that the additive approximation may overlook. These findings help clarify the minimal model for nickelate superconductivity, determine the magnitude and symmetry of their superconducting gaps, and provide a general theoretical framework for Raman experimental analysis of other multi-orbital superconductors, such as iron-based superconductors.

This review summarizes recent advances in nickelate superconductors, covering infinite-layer, bilayer, and trilayer systems, their superconducting properti

By systematically tuning the oxygen content of La₃Ni₂O₇₊δ samples, this study synthesized materials with varying phase compositions, including pure bilayer phase, a mixed phase of bilayer and monolayer–bilayer hybrid, and a predominant bilayer phase containing trilayer intergrowths. High-pressure transport measurements revealed that these phases correspond to distinct superconducting transition temperatures (T_c), with the bilayer phase exhibiting superconductivity at approximately 80 K, while the hybrid and trilayer-intergrowth phases show lower T_c values. Oxygen content not only influences phase purity but also directly modulates the upper critical field (H_c2) of the bilayer superconductivity, with the pure bilayer phase displaying a higher H_c2. By constructing a phase diagram of T_c and H_c2 as functions of oxygen content, this study achieves precise control over oxygen stoichiometry in Ruddlesden–Popper nickelates, providing critical experimental insights for understanding the high-pressure superconducting mechanism.

In response to the critique by Korolev and Talantsev regarding the calculation of the superconducting phase fraction in the Nature paper by Li et al., the authors provide a point-by-point rebuttal: first, experimental confirmation shows that the weak upturn at low temperatures originates from the background, and no paramagnetic Meissner effect is observed, validating the use of field-cooled data for calculating the superconducting phase fraction; second, the demagnetization effect must be based on the actual variation of measured magnetic moment with the superconducting phase fraction f, whereas Korolev et al. erroneously treated the demagnetizing field as a constant, causing their formula to underestimate f by approximately two-thirds (by a factor close to 1/3), which explains why their calculated result is only about one-third of the reported value (approximately 62.1%); finally, multiple characterizations of the sample (energy-dispersive X-ray spectroscopy, X-ray diffraction, nuclear quadrupole resonance, scanning transmission electron microscopy, etc.) confirm it to be a homogeneous, high-quality bulk single crystal without multiple discrete superconducting regions. Therefore, the method for calculating the superconducting phase fraction in Li et al.’s Nature paper has not been invalidated by Korolev et al.’s analysis.

The recent discovery of a superconductivity signature in La3Ni2O7-δ under a pressure of 14 GPa, with a superconducting transition temperature of around 80 K, has attracted considerable attention. An important aspect of investigating electronic structures is discerning the extent to which the electronic ground state of La3Ni2O7-δ resembles the parent state of the cuprate superconductor, a charge transfer insulator with long-range antiferromagnetism. Through X-ray absorption spectroscopy, we reveal the influence of oxygen ligands on the electronic ground states of the Ni ions, displaying a charge transfer nature akin to cuprate but with distinct orbital configurations. Additionally, in La3Ni2O7-δ films, we detect a superlattice reflection (1/4, 1/4, L) at the Ni L absorption edge using resonant X-ray scattering measurements. Further examination of the resonance profile indicates that the reflection originates from the Ni d orbitals. By evaluating the reflection’s azimuthal angle dependence, we confirm the presence of collinear antiferromagnetic spin ordering and charge-like anisotropy ordered with the same periodicity. Our findings reveal a microscopic relationship between these two components in the temperature dependence of the scattering intensity of the reflection. This investigation enriches our understanding of high-temperature superconductivity in La3Ni2O7-δ under high pressure.

This study employs the determinant quantum Monte Carlo method to simulate the low-energy electronic structure of infinite-layer nickelates by adding a gap s orbital with three-dimensional dispersion to the three-orbital Emery model. Large-scale calculations reveal that strong correlation effects significantly reduce the electron pocket induced by the gap s orbital, yet the pocket persists at 20% hole doping, with a size comparable to ARPES experimental observations; the d_{x^2-y^2} orbital dispersion undergoes strong renormalization, and the weak dispersion along the k_z direction agrees with experiments. Furthermore, compared to the conventional three-orbital model, the introduction of the s orbital markedly enhances short-range antiferromagnetic correlations. These results highlight the crucial role of strong correlation and multi-orbital effects in determining the low-energy electronic states and spin correlations of infinite-layer nickelates, indicating that interaction-driven many-body physics must be treated within a realistic multi-orbital framework.

This study investigates the interplay of spin density wave (SDW) and charge-anisotropy-like order with superconductivity in bilayer nickelate films by combining ozone annealing and isovalent rare-earth A-site substitution to effectively apply c-axis uniaxial pressure and tune interlayer coupling. Resonant elastic X-ray scattering shows that, upon optimizing superconductivity, the SDW transition temperature decreases from 150 K to 70 K; although weakened, the SDW persists, whereas the charge-anisotropy-like order is completely eliminated in the superconducting state. Polarization-resolved O K-edge X-ray absorption spectroscopy and electronic structure calculations indicate that both the c-axis compression induced by A-site substitution and the filling of interlayer oxygen vacancies by ozone annealing enhance interlayer Ni-O-Ni hybridization, reconstruct the Fermi surface, and suppress magnetic fluctuations near the SDW wave vector, thereby weakening the SDW instability. These results indicate that the two electronic orders are governed by different energy scales and are consistent with a spin–spinless stripe ground state, providing new clues for understanding density-wave formation and its relationship to superconductivity in bilayer nickelates.

High-quality single crystals of the trilayer Ruddlesden–Popper nickelate La4Ni3O10 were studied using a diamond anvil cell to explicitly tune the nonhydrostatic pressure component, combined with high-pressure electrical transport and synchrotron X-ray diffraction, thereby distinguishing out-of-plane-dominated compression from enhanced in-plane biaxial compression. Enhanced in-plane biaxial compression induced a uniform, reproducible zero-resistance superconducting state at 17.2 and 18.9 GPa, with the required pressure reduced by about 60% compared with previous reports; meanwhile, the density-wave phase under low compression was completely suppressed and separated from the superconducting state, which the authors interpret as effectively ruling out soft quantum critical fluctuations of the density-wave order parameter as a pairing glue in this system. In contrast, out-of-plane-dominated compression only slowly suppressed the density wave, and zero resistance was still not achieved up to 34 GPa, with superconducting signatures being filamentary and inhomogeneous. High-pressure XRD showed that the ambient-pressure monoclinic phase transforms into a tetragonal phase under high pressure, whereas an orthorhombic phase appears as an intermediate under enhanced in-plane compression. On this basis, the authors conclude that in-plane biaxial compression is an effective means to tune nickelate superconductivity, unifies the pressure scales of bilayer and trilayer Ruddlesden–Popper structures, and is linked to results on strained thin films.

Ruddlesden-Popper nickel oxides exhibit superconductivity under both high-pressure bulk and thin-film epitaxial constraints, yet this behavior is highly dependent on sample quality, oxygen content, defects, and stress states. This paper proposes that metastable RP lattices enter the superconducting state only when the local constrained deformation of the Ni-O framework falls within a bounded shear strain window; this deformation governs octahedral rotations, interlayer Ni-O-Ni bond angles, and the coupling between Ni dz² and dx²-y² orbitals. This shear-stress-constrained superconductivity (SSCS) framework unifies previously observed phenomena such as pressure thresholds, reversibility, spatial inhomogeneity, pressure medium dependence, film-substrate sensitivity, and reproducibility challenges. The SSCS scenario does not replace the role of traditional factors such as bond angles, bond lengths, orbital occupancy, oxygen stoichiometry, or carrier density, but rather identifies the mechanical and symmetry conditions required for these factors to cooperatively stabilize the superconducting state. The brittleness and heterogeneity observed in nickel oxide superconductors are not extrinsic complexities but rather core diagnostic features of the superconducting state itself. This perspective provides specific experimental pathways for improving reproducibility and unifies the physical mechanisms underlying compressed bulk materials, epitaxial films, chemically substituted samples, and hybrid RP structures within a single conceptual framework.

The discovery of high-temperature superconductivity near 80 K in bilayer nickelate La3Ni2O7 under high pressures has renewed the exploration of superconducting nickelate in bulk materials. The extension of superconductivity in other nickelates in a broader family is also essential. Here, we report the experimental observation of superconducting signature in trilayer nickelate La4Ni3O10 under high pressures. By using a modified sol-gel method and post-annealing treatment under high oxygen pressure, we successfully obtained polycrystalline La4Ni3O10 samples with different transport behaviors at ambient pressure. Then we performed high-pressure electrical resistance measurements on these samples in a diamond-anvil-cell apparatus. Surprisingly, the signature of possible superconducting transition with a maximum transition temperature (T c) of about 20 K under high pressures is observed, as evidenced by a clear drop of resistance and the suppression of resistance drops under magnetic fields. Although the resistance drop is sample-dependent and relatively small, it appears in all of our measured samples. We argue that the observed superconducting signal is most likely to originate from the main phase of La4Ni3O10. Our findings will motivate the exploration of superconductivity in a broader family of nickelates and shed light on the understanding of the underlying mechanisms of high-T c superconductivity in nickelates.