Knowledge

This study used Ni M1-edge s-core-level nonresonant inelastic X-ray scattering (s-NIXS), a bulk-sensitive method, to determine the angular distribution of Ni 3d holes in the ambient-pressure single-crystal bilayer nickelate La2SmNi2O7. Fitting yielded an e_g hole occupancy ratio h_z/h_x = 0.94 ± 0.06, significantly higher than the 2/3 expected from the model of a fully occupied d_{z^2} bonding band and close to the picture of approximately one hole per e_g orbital under Hund coupling, and consistent with DMFT charge-transfer calculations for the high-pressure phase. On this basis, it is argued that the superconducting pairing scenario for this material should allow both e_g orbitals to participate in electron pairing rather than being dominated solely by a single d_{z^2} orbital; however, this constitutes experimental support for the relevant pairing theory and does not directly establish the pairing mechanism.

Disorder-Induced Suppression of Superconductivity in Infinite-Layer Nickelates

This study employs resonant inelastic X-ray scattering (RIXS) to directly compare the electronic and magnetic excitation properties of trilayer nickelate La₄Ni₃O₁₀ and bilayer La₃Ni₂O₇. The results show that La₄Ni₃O₁₀ exhibits more itinerant behavior, evidenced by broader Ni d-d orbital excitations and a stronger fluorescence background, indicating weaker electronic correlations than in the bilayer system. Despite the weaker correlations, clear collective spin excitations are observed, including dispersive acoustic and optical magnon branches as well as incommensurate spin density waves (SDW). Using linear spin-wave theory analysis, the interlayer superexchange interaction Jz is extracted to be approximately 22 meV, significantly smaller than that in La₃Ni₂O₇. The weaker electron correlations and reduced interlayer magnetic exchange together account for the substantially lower superconducting transition temperature of the trilayer compound (about 30 K) compared to the bilayer (about 80 K). This study establishes interlayer magnetic coupling and electronic correlations as key parameters for superconductivity in layered nickelates, providing important constraints for understanding the superconducting pairing mechanism in this emerging family.

11 linked papers

7 linked papers

Using muon spin rotation/relaxation (μSR), this study systematically measured a series of infinite-layer nickelate (La,Sr)NiO₂ samples spanning from the parent compound through the superconducting dome to the overdoped regime (Sr doping 0% ≤ x ≤ 25%) to probe the magnetic ground state and temperature-dependent static and dynamic magnetism. The results show that, regardless of doping level, local magnetic moments undergo spin freezing at temperatures on the order of tens of kelvin and enter a glassy state, and no anomaly is observed near the superconducting onset, indicating that the magnetism is intrinsic and essentially independent of hole concentration. With increasing hole doping, the glassy state shows only a weak tendency toward destabilization. These observations suggest that magnetism and superconductivity in nickelates are largely decoupled, and that their indirect interactions need to be understood within a multiorbital framework.

Using Ni (L_3)-edge resonant inelastic X-ray scattering (RIXS), this work systematically investigates the evolution of electronic and spin excitations with carrier doping in coherently compressively strained La(_{3-x})Sr(_x)Ni(_2)O(_7)/SrLaAlO(_4) thin films, covering the superconducting ((x = 0, 0.09, 0.21)) and overdoped non-superconducting ((x = 0.38)) regimes. In the superconducting films, dispersive spin excitations persist along the ([H,H]) and ([H,0]) directions, with the dispersion remaining almost doping-independent and exhibiting minimal damping, while the spectral weight only moderately decreases, indicating robust bistripe spin correlations. However, in the non-superconducting film at (x = 0.38), the magnetic response becomes strongly broadened and weakened, accompanied by significantly enhanced damping and a spectral weight reduction of approximately 50%, signaling the collapse of coherent bistripe spin excitations. The simultaneous disappearance of magnetic coherence with superconductivity directly establishes the link between doping-controlled magnetism and superconductivity in layered nickelate thin films.

Using a minimal bilayer two-orbital model and the random phase approximation, we investigate the superconducting pairing symmetry and its doping evolution in pressurized La₃Ni₂O₇. In the undoped system, the most favorable pairing state is an s± wave whose gap function changes sign between different Fermi pockets; the analysis indicates that this unconventional pairing arises from repulsive interactions mediated by the magnetic odd mode of the bilayer nickelate. Hole doping enlarges the γ Fermi pocket, and the intrapocket repulsion driven by the magnetic even mode gradually strengthens and dominates the pairing interaction, eventually driving the pairing symmetry from s± wave to dxy wave in the heavily hole-doped regime. In contrast, under electron doping the s±-wave pairing persists and remains stable even in the deeply electron-doped region where a Lifshitz transition occurs, indicating that the γ Fermi pocket is not a necessary condition for superconductivity in bilayer nickelates; owing to favorable nesting between the α and β pockets that enhances spin fluctuations, the s± wave becomes even more robust in the absence of the γ pocket. This study reveals the doping control of pairing symmetry and provides a new route for testing the superconducting pairing mechanism in pressurized La₃Ni₂O₇.

2 linked papers

Through muon spin rotation/relaxation and resistivity measurements combined with oxygen isotope substitution, the pressure and isotope effects on density wave transitions in the trilayer Ruddlesden-Popper nickelate La₄Ni₃O₁₀ were systematically investigated. Under ambient pressure, two incommensurate spin density wave (SDW) transitions were observed at 132 K and 80–90 K; the magnetic structure reveals that the outer two Ni layers exhibit an antiferromagnetically coupled SDW order, while the inner layer has a smaller magnetic moment, and a c-axis component of the magnetic moment emerges below T*. The abrupt onset of the internal field at T_SDW indicates that the SDW transition resembles a first-order phase change and is closely intertwined with the charge density wave (CDW) occurring at the same temperature. Under applied pressure, T_SDW, T*, and T_CDW are uniformly suppressed at a rate of approximately -13 K/GPa, differing from the behavior in bilayer La₃Ni₂O₇ where pressure increases the separation between SDW and CDW. Substitution of ¹⁶O with ¹⁸O raises T_CDW; in the region where CDW and SDW are intertwined, T_SDW also exhibits a significant isotope effect similar in magnitude to the shift in T_CDW, whereas no isotope effect is observed for the SDW at T* where it evolves independently. These results reveal the strongly intertwined nature of SDW and CDW in La₄Ni₃O₁₀ and suggest that pressure-induced suppression of the CDW order may be a key mechanism for high-pressure superconductivity in Ruddlesden-Popper nickelates.