Paper

Ruddlesden-Popper (RP) nickelates are a promising class of high-temperature superconductors, with superconducting transition temperatures exceeding the boiling point of liquid nitrogen. However, oxygen nonstoichiometry remains a persistent challenge that commonly presents in all RP nickelates. Understanding the formation of oxygen vacancies, their ordering patterns, and their impact on superconductivity is crucial, especially in the newly discovered L⁢a4⁢N⁢i3⁢O10. In this study, the first-principles structural calculations reveal the formation of in-plane oxygen vacancy chains in L⁢a4⁢N⁢i3⁢O10, a key structural feature observed under both ambient and pressurized conditions. These vacancies induce significant lattice distortion and generate residual electrons that hybridize with the Ni 𝑑𝑧2 orbital, altering the sign of the hopping integral between the 𝑑𝑧2 orbitals. Furthermore, the vacancies alter the N⁢i2+/N⁢i3+ ratio, lower the 𝑑𝑧2 orbital energy, and decrease the 𝑑𝑧2 orbital density of states at the Fermi level. Interestingly, at higher vacancy concentrations, the vacancy chains tend to align diagonally along the out-of-plane direction. The phase diagram of L⁢a4⁢N⁢i3⁢O10 exhibits a narrow stability range at ambient pressure, which expands under applied pressure, aligning with the high-oxygen-pressure conditions required for its synthesis. Importantly, these vacancy chains broaden the optical conductivity peak, which could serve as a marker for their detection. Our findings offer valuable insights into the distribution of oxygen vacancies, their role in modifying the electronic structure, and their influence on optical conductivity in L⁢a4⁢N⁢i3⁢O10.

The discovery of high-temperature superconductivity in bulk La3Ni2O7 under high hydrostatic pressure1−4 and biaxial compression in epitaxial thin films5−8 has ignited significant interest in understanding the interplay between atomic and electronic structure in these compounds. Subtle changes in the nickel-oxygen bonding environment are thought to be key drivers for stabilizing superconductivity, but specific details of which bonds and which modifications are most relevant remains so far unresolved. While direct, atomic-scale structural characterization under hydrostatic pressure is beyond current experimental capabilities, static stabilization of strained La3Ni2O7 films provides a platform well-suited to investigation with new picometer-resolution electron microscopy methods. Here, we use multislice electron ptychography (MEP)9,10 to directly measure the atomic-scale structural evolution of La3Ni2O7 thin films across a wide range of biaxial strains tuned via substrate choice. By resolving both the cation and oxygen sublattices, we study the strain-dependent evolution of atomic bonds, providing the opportunity to isolate and disentangle the effects of specific structural motifs for stabilizing superconductivity. We identify the lifting of crystalline symmetry through modification of the nickel-oxygen octahedral distortions under compressive strain as a key structural ingredient for superconductivity and identify in-plane lattice compression as a common attribute between bulk and thin film superconductivity. Building upon the detailed structures obtained by MEP, we introduce a theoretical framework to disentangle coupled structural distortions in corner-sharing octahedra11, which suggest that both known superconducting geometries of La3Ni2O7 (hydrostatic pressure and compressive strain) suppress local t2g orbital mixing in the low-energy Ni bands by raising the octahedral symmetry.

This study systematically investigates the structural stability, electronic structure, and magnetic properties of the alternating monolayer-trilayer (1313) stacked La₃Ni₂O₇ polymorphs using first-principles calculations and group theory analysis. At ambient pressure, the highest-symmetry Cmmm structure exhibits multiple unstable phonon branches at high-symmetry points in the Brillouin zone, and the corresponding distortions can lead to another experimentally reported space group Imma, which features NiO₆ octahedral tilting. Magnetic analysis indicates that the electronic structure of this material at ambient pressure is predominantly governed by the trilayer block, whereas the monolayer block is in a Mott insulating state. Under pressure, the tetragonal P4/mmm structure becomes stable, consistent with experimental observations. The study reveals that octahedral tilting is not a prerequisite for superconductivity and elucidates the symmetry relationships among different space groups as well as the pressure-driven structural phase transition mechanism.

Using the functional renormalization group method, this study investigates the competition between spin-density wave order and superconductivity in the bilayer nickelate La₃Ni₂O₇ under both ambient and high-pressure crystal structures. By comparing weakly coupled multi-orbital models of the two structures, it is found that as the Hund coupling increases, the dominant instability transitions from superconductivity to a spin-density wave with a characteristic wave vector Q₁≈(π/2,π/2), consistent with experiments. Surprisingly, the non-interacting susceptibilities and fRG leading instabilities are nearly identical for the ambient and high-pressure structures, indicating that the emergence of superconductivity under pressure cannot be solely attributed to changes in low-energy electronic structure. Further analysis reveals that suppressing orthorhombic distortion is key: when the system approaches the tetragonal limit, symmetry-related spin-density wave fluctuations become nearly degenerate, thereby hindering long-range magnetic order and enhancing pairing interactions. These results highlight lattice symmetry as a crucial parameter in tuning the competing ordered states in bilayer nickelates and suggest that reducing orthorhombic distortion through uniaxial strain may enable bulk superconductivity at ambient pressure.

Atomic structure and electronic band structure are fundamental properties for understanding the mechanism of superconductivity. Motivated by the discovery of pressure-induced high-temperature superconductivity at 80 K in the bilayer Rud-dlesden-Popper nickelate La3Ni2O7, the atomic structure and electronic band structure of the trilayer nickelate La4Ni3O10 under pressure up to 44.3 GPa are investigated. A structural transition from the monoclinic P21/a space group to the tetragonal I4/mmm around 12.6–13.4 GPa is identified, accompanied by a drop of resistance below 7 K. Density functional theory calculations suggest that the bonding state of Ni $$3{d_{{z^2}}}$$orbital rises and crosses the Fermi level at high pressures, which may give rise to possible superconductivity observed in resistance under pressure in La4Ni3O10. The trilayer nickelate La4Ni3O10 shows some similarities with the bilayer La3Ni2O7 and has unique properties, providing a new platform to investigate the underlying mechanism of superconductivity in nickelates.

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.

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.

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.