Spectroscopic Evidence of Competing Diagonal Spin Interactions and Spin Disproportionation in the Bilayer Nickelate La₃Ni₂O₇

摘要

该研究利用常压拉曼散射系统测量了双层镍酸盐 La₃Ni₂O₇ 的电子、磁性和晶格激发。进入 153 K 以下的自旋密度波态后，A₁g 通道出现突变的电子谱隙及约 50 meV 的等吸收点，而 B₁g 和 B₂g 通道由显著的双磁振子激发主导，表明沿 Ni–O 键及其对角方向存在两种面内自旋交换相互作用。基于自旋波理论的计算进一步指出，B₂g 双磁振子模式源于由镍 d_x²−y² 轨道介导的两个键对角反铁磁相互作用之间的竞争；低于 10 meV 的低能双磁振子激发则来自较弱的自旋矩，支持自旋歧化。研究还发现 B₁g 声子从 280 K 到 4.5 K 的异常软化，提示存在导致棋盘状呼吸调制的初生晶格不稳定性。这些结果将竞争性键对角相互作用、自旋歧化和晶格不稳定性联系起来，为理解镍酸盐超导机理提供了关键要素。

材料

• La₃Ni₂O₇

方法

• Raman scattering
• spin wave theory calculations
• random phase approximation (RPA) renormalization
• Holstein-Primakoff transformation
• Bogoliubov transformation
• powder x-ray diffraction (XRD)
• STEM
• x-ray Laue diffraction
• focused ion beam (FIB)

关键词

• spin density wave
• two magnon excitations
• spin disproportionation
• incipient lattice instability
• checkerboard type breathing modulation
• spin phonon coupling
• spin frustration
• fleury loudon exchange scattering
• bond diagonal antiferromagnetic interactions
• mottness

亮点

• First spectroscopic evidence for two distinct intralayer exchange interactions: along the Ni-O bond direction and along the diagonal.
• The simultaneous observation of prominent two-magnon peaks in both B1g and B2g channels marks a key feature distinguishing La₃Ni₂O₇ from cuprate superconductors.
• The presence of bond-diagonal spin exchange interactions marks a key distinction between the magnetism of La₃Ni₂O₇ and that of the cuprates.
• The pronounced two-magnon signals indicate that La₃Ni₂O₇ is positioned closer to the localized, Mott-insulating limit compared to other layered nickelates such as La₄Ni₃O₁₀.
• The B1g phonon softening exceeds 1.5 meV, over 25% of its room-temperature frequency and an order of magnitude larger than shifts in other phonons, indicating a lattice softening driven by a separate degree of freedom.
• The dynamic Ni-O bond disproportionation provides a natural microscopic mechanism for the site-selective magnetic state by favoring inequivalent long and short Ni-O bond environments.
• The two-magnon features are robust against applied magnetic fields up to 7 T and reproduced at a different excitation wavelength, confirming intrinsic Raman scattering signals.

结论

• The A1g channel exhibits an abrupt electronic spectral gap with an isosbestic point near 50 meV upon entering the spin density wave state below 153 K.
• The B1g and B2g channels are dominated by pronounced two-magnon excitations, providing evidence for two distinct in-plane spin exchange interactions along the Ni-O bonding and its diagonal directions.
• The B2g two-magnon mode arises from competition between two bond-diagonal antiferromagnetic interactions mediated by nickel d_x²-y² orbitals.
• Emergent low-energy two-magnon excitations below 10 meV originate from distinct, weaker spin moments, supporting spin disproportionation with the weaker moment approximately an order of magnitude smaller than the stronger one.
• An anomalous softening of B1g phonons from 280 down to 4.5 K suggests an incipient lattice instability leading to checkerboard-type breathing modulations that remain dynamic without freezing.
• The ground state of the bilayer nickelate is characterized by competing bond-diagonal interactions, spin disproportionation, and an incipient lattice instability, establishing key ingredients for understanding nickelate superconductivity.

主要论断

• Below the spin density wave transition at 153 K, the A1g Raman channel shows an abrupt electronic spectral gap with an isosbestic point near 50 meV.
  • 证据: low-energy spectral weight below 50 meV sharply suppressed below T_SDW,low-energy dynamic susceptibility difference decreases sharply below T_SDW,reproduced in additional crystals (Samples 3 and 4)
• Prominent two-magnon excitations in both B1g and B2g channels provide evidence for two distinct in-plane spin exchange interactions along the Ni-O bond and diagonal directions.
  • 证据: sharp B2g peak at 47 meV with about 37% of B1g intensity,spin-wave plus Fleury-Loudon calculations reproduce peak energies, lineshapes, and relative intensities,exclusion of phonon, single-magnon, and interband origins
• The B2g two-magnon mode arises from competition between two bond-diagonal antiferromagnetic interactions mediated by Ni d_x²-y² orbitals.
  • 证据: model requires two diagonal AF interactions J2 and J2' to reproduce lineshape,spectrum not reproduced if J2' is absent or ferromagnetic,magnetic structure unstable if J2' exceeds J2, constraining J2' about twice J2
• Emergent low-energy two-magnon excitations below 10 meV originate from weaker spin moments, supporting spin disproportionation.
  • 证据: low-energy features reproduced by low-energy magnon branches below 5 meV,features vanish in calculations assuming uniform spin moments,consistent with muSR and NPD reports of coexisting spin moments
• Anomalous softening of B1g phonons from 280 K to 4.5 K indicates an incipient lattice instability toward checkerboard-type breathing modulations.
  • 证据: B1g phonon energy decreases by more than 1.5 meV, over 25% of room-temperature frequency,softening is continuous across T_SDW and insensitive to magnetic ordering,soft mode remains at finite frequency, consistent with dynamic rather than static disproportionation
• The magnetism of La₃Ni₂O₇ differs from cuprates because of significant bond-diagonal exchange interactions and proximity to the Mott-insulating limit.
  • 证据: strong two-magnon signal in B2g channel absent in cuprates,prominent two-magnon modes despite metallicity, unlike more metallic trilayer nickelate,resistivity comparable to or higher than typical metallic cuprates

研究流程

• sample_preparation — High-quality single-domain La₃Ni₂O₇ crystals suitable for symmetry-resolved Raman spectroscopy were obtained.
  • 材料: La₃Ni₂O₇ single crystals grown by optical floating-zone furnace under 15 bar oxygen; fresh mechanically cleaved crystal surfaces
  • 方法: optical floating-zone crystal growth; mechanical cleavage; powder XRD for crystallinity; STEM (Cs-corrected JEM-ARM200F, 200 kV) for microstructure; X-ray Laue diffraction for orthorhombic orientation
  • 观察: crystals refined to structure with minor intergrowths of other Ruddsten-Popper phases; single orthorhombic domain confirmed on millimeter scale by Laue; consistent polarization dependence across samples
• measurement — Symmetry-resolved Raman spectra reveal electronic gap, multichannel two-magnon excitations, and anomalous phonon softening in La₃Ni₂O₇.
  • 材料: La₃Ni₂O₇ crystals (Samples 1-4)
  • 方法: polarized micro-Raman scattering (532 nm, XperRam200); macro-Raman with triple spectrometer (514.5 nm) under magnetic field; phonon peak fitting and subtraction (Lorentzian profiles)
  • 观察: A1g channel shows abrupt electronic gap with isosbestic point near 50 meV below T_SDW = 153 K; B1g and B2g channels show prominent two-magnon peaks at 81 and 47 meV; low-energy two-magnon humps below 15 meV emerge below 30 K; B1g phonon softens from 280 K to 4.5 K by more than 1.5 meV
• analysis — The observed B1g and B2g peaks are two-magnon excitations arising from distinct in-plane exchange interactions along Ni-O bonds and diagonal directions.
  • 材料: phonon-subtracted Raman response data
  • 方法: spin-wave theory with 8-sublattice bilayer antiferromagnetic Heisenberg Hamiltonian; Holstein-Primakoff and Bogoliubov transformation; Fleury-Loudon exchange scattering formalism; RPA-type renormalization of two-magnon response; systematic exclusion of alternative origins (phonon, single-magnon, interband)
  • 观察: calculated spectra reproduce peak energies, lineshapes, and relative B1g/B2g intensities at 4.5 K; low-energy two-magnon peaks vanish for uniform-moment stripe models; competition of two diagonal AF interactions required to reproduce lineshape; B2g mode requires J2' approximately twice J2
• interpretation — The ground state of La₃Ni₂O₇ combines competing bond-diagonal interactions, spin disproportionation, and incipient lattice instability, key ingredients for nickelate superconductivity.
  • 材料: combined Raman, spin-wave, and literature muSR/NPD/RIXS/XAS data
  • 方法: comparison with cuprates and other layered nickelates; correlation of magnetic and lattice anomalies
  • 观察: two competing diagonal AF interactions and on-axis interaction coexist; low-energy two-magnon modes indicate weaker spin moments (spin disproportionation); soft B1g phonon indicates incipient lattice instability toward checkerboard breathing modulation