Knowledge

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctua…

The discovery of superconductivity with an onset temperature of ∼80 K in pressurized bilayer Ruddlesden-Popper La3Ni2O7-δ has attracted much attention. Despite intense research, determination of the exact oxygen content and understanding of the relationship between superconductivity and oxygen content remain a big challenge. Here, we report a systematical study on the structure and physical properties of La3Ni2O7-δ polycrystalline powders which were prepared using the sol-gel method at ambient pressure and then annealed under high oxygen pressure (pO2) or in ozone. The superconducting transition of La3Ni2O7-δ at ∼80 K under high pressure is suppressed for high pO2 and ozone annealed samples. We attribute this to the combination of the following two reasons: (i) damage of the bilayer structure, as revealed by powder X-ray diffraction, scanning transmission electron microscopy and pair distribution function measurements, and (ii) hole overdoping due to the increasing of oxygen content. Our results reveal that the bilayer structure in La3Ni2O7-δ is fragile and post-annealing under mild oxidization is suitable for maintaining the integrity of the bilayer structure and increasing oxygen content.

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.

11 linked papers

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 first-principles calculations, this study investigates the structural stability, electronic structure, and magnetism of the newly synthesized double-infinite-layer oxyfluoride La₃Ni₂O₅F, along with the effects of chemical pressure and epitaxial strain. The phonon spectrum confirms the dynamical stability of the crystal structure, while the electronic structure exhibits a highly two-dimensional, cuprate-like Fermi surface dominated by Ni-d_(x²−y²) states, with an effective filling of approximately d^1.2 resulting from self-doping by rare-earth ions, and a charge-transfer energy close to the cuprate regime. These electronic features remain remarkably robust under chemical pressure and epitaxial strain, with only moderate changes in the Ni-d_(x²−y²) filling. Spin-polarized calculations reveal multiple near-degenerate magnetic configurations with various in-plane and out-of-plane spin arrangements, and compressive strain further enhances magnetic frustration without significantly altering the electronic structure. Consequently, La₃Ni₂O₅F is established as a promising cuprate analog, and lattice engineering can effectively provide fine-tuning of its electronic and magnetic properties.

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.

12 linked papers

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.