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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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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 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.

This work employs resonant inelastic X-ray scattering (RIXS) to systematically investigate the electronic and spin excitations in La₃Ni₂O₇ (LNO) thin films under epitaxial strain spanning from approximately –2% to +1.9%. In compressively strained LNO/SrLaAlO₄ films that exhibit ambient-pressure superconductivity with an onset critical temperature above 40 K, dd excitations and spin dynamics resembling those of bulk LNO are observed, yet the spin excitation bandwidth increases by about 10 meV, indicating an enhanced interlayer antiferromagnetic exchange coupling Jz; conversely, tensile-strained LNO/SrTiO₃ films display a pronounced suppression of both spin excitations and Ni 3dz²-related dd excitations. This evolution reflects how strain tunes the Ni 3dz²–O 2pz hybridization and the interlayer distance, thereby modulating the interlayer magnetic coupling strength. The results demonstrate that epitaxial strain effectively controls the interlayer antiferromagnetic superexchange in bilayer nickelates, and the strengthened Jz is closely correlated with the emergence of ambient-pressure superconductivity, lending support to the theoretical picture in which interlayer magnetic exchange facilitates interlayer pairing.

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