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Very recently, a new superconductor with Tc = 80 K has been reported in nickelate (La3Ni2O7) at around 15–40 GPa conditions (Nature, 621, 493, 2023), which is the second type of unconventional superconductor, besides cuprates, with Tc above liquid nitrogen temperature. However, the phase diagram plotted in this report was mostly based on the transport measurement under low-temperature and high-pressure conditions, and the assumed corresponding X-ray diffraction (XRD) results were carried out at room temperature. This encouraged us to carry out in situ high-pressure and low-temperature synchrotron XRD experiments to determine which phase is responsible for the high Tc state. In addition to the phase transition from the orthorhombic Amam structure to the orthorhombic Fmmm structure, a tetragonal phase with the space group of I4/mmm was discovered when the sample was compressed to around 19 GPa at 40 K where the superconductivity takes place in La3Ni2O7. The calculations based on this tetragonal structure reveal that the electronic states that approached the Fermi energy were mainly dominated by the eg orbitals (3dz2 and 3dx2–y2) of Ni atoms, which are located in the oxygen octahedral crystal field. The correlation between Tc and this structural evolution, especially Ni–O octahedra regularity and the in-plane Ni–O–Ni bonding angles, is analyzed. This work sheds new light to identify what is the most likely phase responsible for superconductivity in double-layered nickelate.

139 La-NMR/NQR measurements of La 3 Ni 2 O 7-δ (δ∼0.0 and δ∼0.08), and La 4 Ni 3 O 10 have been performed. 139 La-NMR and transport and magnetic studies have also been carried out for Tl(La 2 Sr 2 )Ni 2 O 9 . Anomalous temperature ( T ) dependence of the longitudinal relaxation rates 1/ T 1 has been found at temperatures T A ∼(140–150 K) for all the systems. In Tl(La 2 Sr 2 )Ni 2 O 9 , there exists a transition to a magnetically ordered state at T ∼20 K. The large broadening of the NMR spectra observed for Tl(La 2 Sr 2 )Ni 2 O 9 below 20 K indicates that the system is in the charge ordered state in the temperature region between 20 K and T A with localized magnetic moments at Ni sites. The NQR intensity of La 4 Ni 3 O 10 begins to decrease rapidly with decreasing T at T A ∼140 K and almost disappears at T ∼120 K (wipeout). By arguing results of the present experimental studies, we propose that in all the systems with NiO 2 planes studied here exhibit similar type transitions to the charge ordered states at temperatures T A , all of which are in the narrow T region around 140 K. It has also been found that all the systems exhibit resistivity anomalies in the T region of (450–550) K, which suggests that they have a tendency of similar type changes or transitions of their electronic states at the temperatures, too.

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Researchers successfully synthesized a series of infinite-layer Nd₁₋ₓEuₓNiO₂ thin films with doping extended to x = 0–0.7 using pulsed laser deposition combined with calcium hydride topotactic reduction. Electrical transport measurements reveal a superconducting dome in the range 0.2 ≤ x ≤ 0.5, whose doping width is larger than that of samples fabricated by molecular beam epitaxy and comparable to that achieved by chemical solution methods. The film with x = 0.3 exhibits an optimal superconducting transition temperature of about 31 K, significantly higher than values obtained by other vacuum epitaxy techniques, indicating that pulsed laser deposition is an effective route for preparing high-quality, high-transition-temperature nickelate superconducting thin films. Magnetotransport experiments observe robust magnetic-field-enhanced and re-entrant superconductivity in both underdoped and overdoped regions, attributed to the polarization of Eu²⁺ local magnetic moments under an external field that generates an internal exchange field partially compensating the applied field; the Jaccarino-Peter effect alone cannot fully explain this phenomenon, suggesting the existence of additional mechanisms. In the low-temperature region just above the onset superconducting transition temperature, the Hall resistance exhibits a nonlinear character without noticeable magnetic hysteresis, which may arise from magnetic impurity scattering. These results highlight the critical role of magnetic rare-earth Eu²⁺ ions in imparting exotic physical properties to infinite-layer nickelates.

The ambient-pressure superconductivity in La3⁢Ni2⁢O7 thin films via compressive epitaxial strain provides a highly accessible platform for diverse characterization techniques, facilitating the studies of high-temperature superconductivity. Here, we systematically map the phase diagram and reveal the superconducting dome with an electron-hole crossover in compressively strained La3−𝑥⁢Sr𝑥⁢Ni2⁢O7−𝛿 thin films by simultaneously tuning Sr doping and oxygen content. The maximum transition temperature (𝑇𝑐) coincides with an anomalous sign change in the Hall coefficient (𝑅𝐻), reminiscent of electron-doped cuprates, which may signal a Fermi surface reconstruction. Beyond the superconducting dome, a ln⁡1/𝑇 insulating regime and a 𝑇-linear resistivity regime are also resolved, resembling behaviors observed in cuprates and infinite-layer nickelates. This work reveals a dome-shaped relationship between 𝑇𝑐 and 𝑅𝐻 and establishes a key framework for understanding unconventional superconductivity in nickelate systems.

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In 2023, the bilayer nickel oxide La₃Ni₂O₇ was discovered to exhibit superconductivity at a pressure of approximately 14 GPa with a critical temperature near 80 K, featuring a structure similar to that of high-temperature copper oxides; subsequently, superconductivity was also found in the trilayer compound La₄Ni₃O₁₀. These two compounds belong to the Ruddlesden–Popper phase, which consists of alternating stacks of NiO₂ square-lattice layers and LaO rock-salt layers. Current research is mainly pursued along three directions: expanding the chemical diversity of the compounds, raising the superconducting transition temperature through elemental substitution, and elucidating the pairing mechanism of superconductivity. However, key experiments must be conducted under high pressure, which poses difficulties for mechanistic studies; therefore, developing nickel oxides that exhibit superconductivity at lower pressures or even ambient pressure is of great significance. This review summarizes the existing knowledge of these systems, highlights the relatively mature methods for sample synthesis and characterization, and briefly outlines their electronic properties, aiming to provide a foundation for future material exploration and physical understanding of the underlying mechanisms.

This study systematically constructs the superconducting phase diagram of multilayer square-planar nickelates Nd_{n+1}Ni_nO_{2n+2} (n = 4–8), revealing that compounds with n = 4 to 7 exhibit signs of superconductivity, with a maximum onset critical temperature of 12.9 K (n = 6), while n = 8 shows only weak superconducting correlations. As the layer number n decreases, the superconducting anisotropy undergoes a reversal due to the effect of 4f electrons at the neodymium sites—the electronic structure approaches that of cuprates, and magnetic fluctuations persist in both the superconducting region and the overdoped nonsuperconducting region. Notably, this superconducting region overlaps with that of chemically doped infinite-layer nickelates, highlighting commonalities and differences among different structural realizations in square-planar nickelates. This work establishes a general template for synthesizing novel nickel-based superconductors through atomic-precision layered design.

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This study addresses bilayer nickelate films grown on SrLaAlO4(001) substrates (including La2PrNi2O7 and La2SmNi2O7) and explains the incomplete zero resistance and two-step superconducting transitions commonly observed in resistance measurements. Using circular-electrode devices, the azimuthal dependence of the transition was measured, revealing that the two-step feature is directly related to the substrate step–terrace orientation: when the current is parallel to the step edges, a single-step transition appears, whereas when it is perpendicular, the two-step transition is most pronounced; ADF-STEM also directly observed structural damage regions at the steps. On this basis, the films were modeled as intact regions with a higher transition temperature and step weak-link regions with a lower transition temperature, and simple models and finite-element simulations reproduced the experimental features; the lateral dimensions of the structural damage regions are approximately 1–5 nm, comparable to the Ginzburg-Landau in-plane coherence length of approximately 2–3 nm, supporting a Josephson-type weak-link picture. Transport measurements further show that the transition and even the occurrence of superconductivity depend strongly on the average terrace width: samples with average terrace widths of approximately 920 and 540 nm can reach zero resistance, those with approximately 250 nm do not reach zero resistance, and those with approximately 120 nm show no superconductivity. This work identifies step-induced weak links as the microscopic origin of the two-step transition in bilayer nickelate films and provides a framework for understanding anisotropic and percolative transport and low critical currents.