Paper

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.

The spin fluctuations in rare-earth Pr and Nd doped bilayer nickelates La₂LnNi₂O₇₋δ (Ln = La, Pr, Nd) were investigated under ambient pressure using inelastic neutron scattering. In the undoped La₃Ni₂O₇₋δ, a flat spin fluctuation mode at 45 meV was observed; upon doping, this mode splits into two modes at 43 and 48 meV, with an additional weak mode appearing at approximately 60 meV. Notably, the spin fluctuation intensity in La₂NdNi₂O₇₋δ is significantly higher than that in La₃Ni₂O₇₋δ and La₂PrNi₂O₇₋δ. These results are consistent with a description based on the stripe-type antiferromagnetic Heisenberg model, indicating that rare-earth doping enhances the interlayer magnetic coupling, with the interlayer exchange coupling SJ⊥ increasing from about 60 meV to 69–73 meV, while the intralayer coupling remains weak (≤3.5 meV). This enhancement may account for the increase in superconducting transition temperature from 80 K to near 100 K following rare-earth doping. This work reveals the regulatory role of rare-earth doping on spin dynamics and superconducting pairing in bilayer nickelates.

Spin-charge-orbital order in nickelate superconductors

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

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.