Paper

High-pressure studies have revealed superconductivity in La4⁢Ni3⁢O10, sparking interest in its ambient-pressure properties and the underlying electronic correlations. Motivated by experimental observations of an incommensurate spin-density wave (SDW) at ambient pressure, we investigate the SDW characteristics and possible superconductivity in La4⁢Ni3⁢O10 using a multiorbital random-phase approximation (RPA). Starting with a 12-orbital tight-binding model derived from density functional theory (DFT) calculations, we include Hubbard interactions to explore the interplay between electronic correlations and magnetic instabilities. Our analysis reveals a stripe-like SDW with a wave vector 𝐐≈(±0.7⁢𝜋,0), suggesting a possible density wave instability in agreement with experiments. This configuration is driven by nesting between the 𝛼1 pocket, primarily contributed by the outer-layer Ni 𝑑𝑧2 orbitals, and the 𝛽1 pocket, contributed by both the 𝑑𝑧2 and 𝑑𝑥2−𝑦2 orbitals of the outer layer. It exhibits interlayer antiferromagnetic ordering between the top and bottom NiO layers, with the magnetic moment of the middle layer being nearly zero. We demonstrate that the Hund coupling 𝐽𝐻 is the primary driver of the observed SDW and determine the specific criterion: 𝐽𝐻>0.16⁢𝑈. Building upon our findings on the SDW mechanism, we further demonstrate that hole doping (𝛿=−0.4) enhances Fermi surface nesting, leading to the emergence of a superconducting state with a gap structure similar to that of the high-pressure phase.

This study successfully synthesized high-quality, long-range ordered hybrid 1313-type La₃Ni₂O₇ single crystals (with an alternating monolayer-trilayer structure) and systematically characterized their physical properties. At ambient pressure, the material exhibits typical semiconducting behavior and displays distinct anomalies in resistivity, magnetic susceptibility, and specific heat at 170 K. ¹³⁹La nuclear magnetic resonance spectroscopy unambiguously confirms that these anomalies originate from a spin-density wave (SDW) transition. High-pressure electrical transport measurements indicate that the application of pressure can induce metallization; however, no superconductivity is observed up to 65 GPa. These findings establish hybrid 1313-type La₃Ni₂O₇ as a new member of the Ruddlesden-Popper nickelate family featuring a unique SDW transition, offering a platform for investigating the interplay among crystal structure, electronic order, and superconductivity in hybrid nickelates.

Employing density functional theory combined with cluster dynamical mean-field theory, this study investigates bilayer Ruddlesden-Popper nickelates and finds that their physical properties are primarily governed by interlayer “dynamical singlets” formed by single electrons in the 3z²−r² orbital, which hybridize with itinerant x²−y² planar orbitals. This hybridization responds distinctly to hydrostatic pressure and in-plane compressive strain: strain enhances interlayer correlations, leading to an orbitally selective singlet-pairing Mott mechanism, whereas hydrostatic pressure mainly promotes in-plane itinerancy. This difference explains experimental discrepancies between bulk and strained thin films observed in angle-resolved photoemission spectroscopy and transport measurements. The theoretical framework views this low-energy state as a hybridized system of dynamical singlets and itinerant orbitals, offering a new perspective for understanding superconductivity.

Stabilizing and tuning superconductivity in La₃Ni₂O₇₋δ films: Oxygen recycling protocol reveals hole-doping analogue

This work uses scanning SQUID microscopy to investigate the local superfluid density and magnetic structure of an optimally doped infinite-layer nickelate Nd1-xEuxNiO2 (x = 0.25) thin film grown by molecular beam epitaxy on an LSAT substrate. Simultaneous direct-current magnetometry and alternating-current magnetic susceptibility imaging reveal that superconductivity coexists with a weak magnetic texture, and that the superconducting state is highly spatially inhomogeneous: near the superconducting transition, a robust stripe-like enhancement of superconductivity emerges, accompanied by halo-like defect responses and paramagnetic regions between the stripes. These results indicate that competing magnetic phases are the main factor responsible for the anomalous broadening of the superconducting transition in this optimally doped NENO sample; understanding this superconducting enhancement mechanism may provide a route to raising the superconducting temperature of such materials.

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.

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.