Nature of magnetism in bilayer nickelate La₃Ni₂O₇ single crystals

摘要

该研究利用中子散射技术解析了双层镍氧化物La₃Ni₂O₇单晶的自旋有序与动力学。在常压母体相中，于倒空间Q=(0,0.5,2.5)位置观测到清晰的自旋激发，具有约5 meV的自旋能隙和显著的面内各向异性色散——沿横向的带边软化揭示了竞争性交换相互作用。激发表现出双层周期性的面外调制，直接证实了反铁磁层间耦合。基于线性自旋波理论，实验色散可用包含强层间交换与竞争面内耦合的双层海森伯哈密顿量精确描述，磁结构为条纹型有序。将谱强度归一化为绝对单位后发现，尽管自旋波带宽仅为铜氧化物的25%，但在可比能量下局域动态磁化率显著增强，总涨落磁矩与铜氧化物相当。这些结果揭示了源自强电子关联的中能自旋激发是该体系的本质特征，构建了与铜氧化物截然不同的磁性框架，为理解该体系中超导的配对机制提供了直接依据。

材料

• La₃Ni₂O₇

方法

• Neutron scattering
• Inelastic neutron scattering
• Elastic neutron diffraction
• Linear spin wave theory

关键词

• spin excitations
• spin gap
• stripe type magnetic order
• antiferromagnetic interlayer coupling
• magnetic exchange interactions
• spin fluctuations
• magnetic anisotropy

亮点

• First direct evidence of antiferromagnetic interlayer coupling from pronounced out-of-plane modulations in spin excitations.
• Quantitative determination of exchange interactions (Jc, J₁a, J₁b, J₂) using linear spin wave theory.
• Comparison with cuprates reveals a distinct magnetic framework: smaller bandwidth but comparable fluctuating moment suggests dense mid-energy spin fluctuations relevant for pairing.

结论

• Single-crystalline La₃Ni₂O₇ exhibits stripe-type magnetic order with a spin gap of ≈5 meV and anisotropic in-plane dispersions indicative of competing exchange interactions.
• Out-of-plane modulations with bilayer periodicity provide direct evidence for antiferromagnetic interlayer coupling.
• Spin-wave bandwidth is only ≈25% of that in cuprates, but the total fluctuating magnetic moment is comparable, with enhanced mid-energy local dynamic susceptibility.
• The system is well described by a bilayer Heisenberg model with strong interlayer exchange and competing in-plane couplings.

主要论断

• La₃Ni₂O₇ exhibits a stripe-type magnetic ground state with a single magnetic phase (Q = (0,0.5,0.5)) and no phase separation.
  • 证据: Elastic neutron diffraction shows only one magnetic peak at (0,0.5,0.5); no evidence of (0,0.5,0)
• The spin wave dispersion is described by a bilayer Heisenberg model with strong interlayer exchange (SJc ≈40 meV) and competing in-plane couplings, supporting a single-stripe order.
  • 证据: Inelastic neutron scattering data fit well to single-stripe model; double-stripe model fails to reproduce anisotropic dispersion and intensity distribution
• Despite spin-wave bandwidth only 25% of cuprates, local dynamic susceptibility is significantly enhanced at mid energies, and total fluctuating moment is comparable to cuprates.
  • 证据: Absolute normalization gives χ''(ω) integrated moment of 1.75 μ_B²; comparison with cuprate data shows enhanced mid-energy susceptibility

研究流程

• crystal_growth — Millimeter- to centimeter-sized La₃Ni₂O₇ single crystals.
  • 材料: La₂O₃; NiO
  • 方法: solid-state reaction; optical floating-zone growth under O₂ pressure
  • 观察: large high-quality single crystals
• elastic_neutron_diffraction — Antiferromagnetic order with stripe-type structure.
  • 材料: La₃Ni₂O₇ single crystal
  • 方法: triple-axis spectrometer TOPAN at JRR-3
  • 观察: magnetic Bragg peak at Q = (0, 0.5, 2.5); onset at 151K
• inelastic_neutron_scattering — Spin wave dispersion mapped across Brillouin zone.
  • 材料: 25 co-aligned single crystals, total mass 1.26 g
  • 方法: time-of-flight spectrometers ARCS and MAPS; absolute normalization to phonons
  • 观察: spin excitations with gap ≈5 meV; anisotropic dispersion; bilayer periodicity in L
• spin_wave_modeling — Single-stripe order with competing exchange interactions.
  • 材料: experimental dispersion
  • 方法: linear spin wave theory using SpinW; fitting exchange parameters
  • 观察: SJc = 39.86, SJ₁a = 2.36, SJ₁b = 3.71, SJ₂ = 4.63 meV
• absolute_units_normalization — Mid-energy spin excitations are a defining feature of La₃Ni₂O₇.
  • 材料: acoustic phonon normalization
  • 方法: integration of magnetic signal
  • 观察: total fluctuating moment 1.75 μ_B², comparable to cuprates