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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.

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2 linked papers

The study uses La3Ni2O7 single crystals grown by the high-pressure optical floating-zone method and, combining neutron diffraction with inelastic neutron scattering, identifies in the ambient-pressure parent phase a single stripe-type magnetic order at q=(0,0.5,0.5), and measures a spin gap of about 5 meV, in-plane anisotropic excitations, and a pronounced modulation with bilayer periodicity, the latter indicating antiferromagnetic interlayer coupling. The dispersion is described by a bilayer Heisenberg-type model including strong interlayer exchange and competing in-plane exchange; elastic scattering cannot readily distinguish single-stripe from double-stripe order, whereas the inelastic spectra serve as the discriminating basis. Absolute normalization shows that the spin-wave bandwidth is only about 25% that of cuprates, while the local dynamic magnetic susceptibility is enhanced at the corresponding energies, and the total fluctuating moment is comparable to that of cuprates. The results establish a magnetic framework different from that of cuprates and are regarded as relevant to nickelate superconductivity; the paper does not establish a pairing channel, and the role of magnetism in pairing remains a premise to be tested.

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.

This study used an ambient-pressure Raman scattering system to measure the electronic, magnetic, and lattice excitations of the bilayer nickelate La₃Ni₂O₇. Upon entering the spin-density-wave state below 153 K, the A₁g channel exhibits an abrupt electronic spectral gap and an isosbestic point at about 50 meV, whereas the B₁g and B₂g channels are dominated by pronounced two-magnon excitations, indicating two in-plane spin exchange interactions along the Ni–O bond and its diagonal direction. Calculations based on spin-wave theory further indicate that the B₂g two-magnon mode originates from competition between two bond-diagonal antiferromagnetic interactions mediated by the Ni d_{x²−y²} orbitals; the low-energy two-magnon excitations below 10 meV arise from weaker spin moments, supporting spin disproportionation. The study also finds anomalous softening of the B₁g phonon from 280 K to 4.5 K, suggesting an incipient lattice instability that leads to checkerboard-like breathing modulation. These results link competing bond-diagonal interactions, spin disproportionation, and lattice instability, providing key elements for understanding the superconducting mechanism of nickelates.

This study systematically compares the spin and orbital excitation properties of undoped superconducting infinite-layer nickelate PrNiO₂ and insulating cuprate CaCuO₂ using momentum-resolved and polarization-resolved resonant inelastic X-ray scattering (RIXS) measurements. The results show that the in-plane magnetic exchange integral of PrNiO₂ (approximately 46 meV) is significantly smaller than that of CaCuO₂ (approximately 82 meV), while the out-of-plane exchange integrals are similar (approximately 6–7 meV), indicating that both materials support three-dimensional antiferromagnetic order with comparable three-dimensionality of spin-spin correlations. The orbital excitations (intra-3d transitions) are well described by a single-ion model, but the Ni-dxy peak energy is notably lower than that of Cu-dxy, with opposite dispersion directions—nickelate exhibits orbital excitation propagation driven by nearest-neighbor orbital superexchange coupling, whereas cuprate is dominated by next-nearest-neighbor coupling. Despite a significant difference in charge-transfer energy (larger in the nickelate), the spin and orbital excitation characteristics are generally highly similar, with key distinctions only in the energy and dispersion of the Ni-dxy peak, attributed to differing orbital superexchange coupling mechanisms. This work reveals the core commonalities in magnetism and orbital dynamics between infinite-layer nickelates and cuprates, while also indicating smaller spin fluctuation energies and stronger localization of doped charges on metal sites in the nickelates.