Paper

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.

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.

Recently, the bilayer nickelate La3Ni2O7 has been discovered as a new superconductor with transition temperature Tc near 80 K under high pressure1–3. Despite extensive theoretical and experimental work to understand the nature of its superconductivity4–29, the requirement of extreme pressure restricts the use of many experimental probes and limits its application potential. Here we present signatures of superconductivity in La3Ni2O7 thin films at ambient pressure, facilitated by the application of epitaxial compressive strain. The onset Tc varies roughly from 26 to 42 K, with higher Tc values correlating with smaller in-plane lattice constants. We observed the co-existence of other Ruddlesden–Popper phases within the films and dependence of transport behaviour with ozone annealing, suggesting that the observed low zero resistance Tc of around 2 K can be attributed to stacking defects, grain boundaries and oxygen stoichiometry. This finding initiates numerous opportunities to stabilize and study superconductivity in bilayer nickelates at ambient pressure, and to facilitate the broad understanding of the ever-growing number of high temperature and unconventional superconductors in the transition metal oxides.

Signatures of superconductivity near 80 K in a nickelate under high pressure

Using helium as a pressure-transmitting medium, this study constructs the intrinsic pressure–temperature structural phase diagram of stoichiometric bilayer nickelate La₃Ni₂O₇ under hydrostatic pressure via synchrotron single-crystal X-ray diffraction. At ambient pressure, the material adopts a polar orthorhombic Am2m structure, accompanied by charge ordering between inequivalent Ni sites and tilting of NiO₆ octahedra. With increasing pressure, the orthorhombic lattice distortion gradually decreases, while the intensity of superstructure reflections indicative of charge ordering diminishes linearly. At approximately 10 GPa, a direct transition occurs from the charge-ordered Am2m phase to a tetragonal I4/mmm phase, with no intervening Amam phase; orthorhombic twinning splitting disappears, and unit cell parameters approach tetragonal symmetry. This tetragonal phase is characterized by the complete elimination of octahedral tilting, resulting in linear interlayer Ni–O–Ni bonds, and it persists in the pressure–temperature region where superconductivity emerges. This structural transition coincides precisely with the onset of bulk superconductivity at ~68 K as detected in resistance measurements, indicating that the superconducting state resides within the I4/mmm tetragonal framework. These findings resolve controversies over the structural identity of the superconducting phase, establish the intrinsic structural evolution pathway of La₃Ni₂O₇, and provide a critical structural basis for understanding the microscopic origin of high-temperature superconductivity in nickelates.

Researchers integrated soft point-contact Andreev reflection spectroscopy into a palm-type cubic anvil high-pressure cell, employing substrate anchoring and an external wire branching strategy to stably form multiple point-contact junctions under hydrostatic pressures up to 15 GPa. Benchmark measurements on the elemental superconductor Nb verified the method’s reliability and yielded a zero-temperature superconducting energy gap ratio of 2Δ(0)/k_B T_c ≈ 3.3. Further application to the kagome metal superconductor CsCr3Sb5 and the bilayer nickelate superconductor La2PrNi2O7 revealed sharp zero-bias conductance peaks strikingly different from those of conventional BCS superconductors, and their evolution with temperature, magnetic field, and pressure was systematically investigated. Analysis indicates that these spectroscopic features are consistent with unconventional superconductivity and possible d-wave pairing symmetry, providing direct spectroscopic evidence for understanding their pairing mechanism. This work successfully establishes a high-pressure experimental platform that bridges macroscopic electrical transport and microscopic spectroscopic probes, opening a new avenue for broadly exploring the pairing symmetries of pressure-induced unconventional superconductors.