Large-Tiny Spin-Stripe SDW Order in La₃Ni₂O₇ via ¹³⁹La-NQR Spectroscopy: Reconsideration Based on the Am2m Structure

摘要

本研究利用¹³⁹La核四极共振（NQR）光谱重新审视了常压下双层镍氧化物La₃Ni₂O₇的晶体结构与自旋密度波（SDW）态。实验观察到两个不同的La(2)位点，这一特征在低对称性Am2m结构框架下得到自然解释，而此前基于Amam结构的缺陷模型无法自洽。在Am2m结构中，作者将SDW自旋构型重新识别为“大-小”自旋条纹结构，该结构源于两个不等价Ni位点分别具有大和小磁矩。基于DFT+U计算及NQR参数分析，研究证实了该SDW序的巡游性质，指出其由费米面嵌套驱动而非局域磁矩机制，这为理解该材料在高压下高T_c超导性的起源提供了基础。

材料

• La₃Ni₂O₇

方法

• 139La-NQR
• DFT+U

关键词

• spin density wave
• large tiny spin stripe
• itinerant nature
• fermi surface nesting
• am2m structure
• high tc superconductivity

亮点

• The study resolves long-standing inconsistencies in previous NQR analyses based on the Amam structure by demonstrating that both La(2) sites are intrinsic to the Am2m structure.
• The calculated Ni valences from DFT+U (+2.48 and +2.52) show a significantly smaller valence difference than bond-valence-sum analysis, suggesting that the SDW order is not driven by large charge disproportionation.

结论

• The observation of two distinct La(2) sites in the NQR spectra is naturally accounted for by the Am2m structure without any special assumptions, whereas it cannot be reconciled within the Amam framework.
• Within the Am2m framework, the spin configuration of the SDW phase is identified as a large-tiny spin-stripe structure, arising naturally from two inequivalent Ni sites with large and tiny magnetic moments.
• The SDW order in La₃Ni₂O₇ is driven not by a localized-moment mechanism but by itinerant Fermi-surface nesting.
• These results establish the itinerant nature of the SDW order at ambient pressure in La₃Ni₂O₇, providing a basis for understanding its relationship to the emergence of high-T_c superconductivity under high pressure.

主要论断

• The intrinsic ambient-pressure crystal structure of La₃Ni₂O₇ is the lower-symmetry Am2m structure, not the previously assumed Amam structure.
  • 证据: Observation of two distinct La(2) NQR sites (La(2)a and La(2)b) with 1:1 intensity ratio.,DFT+U calculated NQR parameters for Am2m structure match experimental values, while Amam structure predicts only one La(2) site.,Previous defect models (stacking faults, phase contamination, oxygen vacancies) fail to explain the consistent 1:1 intensity ratio and identical relaxation rates.
• The spin-density-wave (SDW) state in La₃Ni₂O₇ is a large-tiny spin-stripe order.
  • 证据: NQR spectral simulations in the SDW phase show small internal field at La(2)a and large internal field at La(2)b.,La(2)a sites are aligned above tiny-spin Nia channels, and La(2)b sites above large-spin Nib channels.,Consistent with recent neutron scattering and 17O-NMR measurements.
• The SDW order in La₃Ni₂O₇ is itinerant, driven by Fermi-surface nesting, rather than by localized magnetic moments.
  • 证据: DFT+U calculations show a small valence difference between Nia and Nib sites (+2.48 vs +2.52), inconsistent with the large magnetic moment difference observed.,In a localized moment picture, the site with higher electron occupancy (Nia) should have a larger moment, but experimentally μ_Nia < μ_Nib.,This inconsistency supports an itinerant mechanism where Fermi-surface nesting drives the order.

研究流程

• Sample Preparation — High-quality single-phase La₃Ni₂O_7.0 sample was prepared for spectroscopic analysis.
  • 材料: Polycrystalline La₃Ni₂O_7+δ (δ=0.0); Solid-state reaction method
  • 方法: Solid-state reaction synthesis; X-ray diffraction (XRD) for phase confirmation; Thermogravimetric analysis for oxygen deficiency determination
  • 观察: Lattice parameters: a = 5.3920 Å, b = 5.4510 Å, c = 20.533 Å; High-quality sample with minimal La₄Ni₃O₁₀ secondary phase (≈4%)
• NQR Measurement — Experimental NQR spectra reveal two distinct La(2) sites with equal occupancy, inconsistent with the single-site Amam structure.
  • 材料: Coarse powder sample of La₃Ni₂O_7.0
  • 方法: 139La Nuclear Quadrupole Resonance (NQR) spectroscopy; Zero external field measurement
  • 观察: Two distinct La(2) NQR peaks observed: La(2)a and La(2)b; Sharp spectral linewidths (60 kHz for La(2)a); Integrated intensity ratio between La(2)a and La(2)b is nearly 1:1
• DFT+U Calculation — DFT+U calculations confirm that the Am2m structure naturally accounts for the observed two La(2) sites and their NQR parameters.
  • 材料: Quantum Espresso package; PBE functional; Hubbard U = 3.5 eV on Ni-3d orbitals
  • 方法: Density Functional Theory plus Hubbard (DFT+U) calculations; Calculation of NQR parameters (ν_Q, η) for Amam, Am2m, and Am2m structures
  • 观察: Calculated NQR parameters for Am2m structure match experimental values for La(2)a and La(2)b; Calculated Ni valences: +2.48 (Nia) and +2.52 (Nib)
• SDW Analysis — The SDW state is identified as a large-tiny spin-stripe structure, where La(2)a couples to tiny-spin Nia sites and La(2)b couples to large-spin Nib sites.
  • 材料: NQR spectra in SDW phase; DFT+U calculated EFG tensors
  • 方法: Simulation of NQR peak shifts due to internal magnetic fields; Analysis of hyperfine coupling between La(2) sites and Ni moments
  • 观察: Asymmetric deformation of NQR spectra below T_SDW; Small internal field at La(2)a and large internal field at La(2)b; Ni moments aligned approximately along c-axis
• Interpretation — The SDW order is driven by itinerant Fermi-surface nesting rather than localized magnetic moments, establishing the itinerant nature of the SDW state.
  • 材料: Experimental NQR data; DFT+U valence calculations; Bond-valence-sum (BVS) analysis from SXRD
  • 方法: Comparison of magnetic moment magnitudes (μ_Nia vs μ_Nib); Comparison of valence contrast from DFT+U vs BVS
  • 观察: μ_Nia ≈ 0.13–0.2 μ_B, μ_Nib ≈ 0.4–0.5 μ_B; DFT+U valence difference (0.04) is much smaller than BVS estimate (0.4); Magnetic moment ratio inconsistent with localized moment expectation based on valence