Contrasting Spin Excitations in Octahedral and Square-Planar n=8 Ruddlesden-Popper Nickelates

摘要

利用Ni L₃边共振非弹性X射线散射（RIXS），本研究对比了八面体Ruddlesden-Popper（RP）相Nd₉Ni₈O₂₅（非超导）及其还原后的平面相Nd₉Ni₈O₁₈（具有约5 K的超导关联）中的低能自旋激发。结果显示，八面体相呈现自旋密度波（SDW）基态，有序波矢为（1/4,1/4），其低能谱由沿(0,π)和(π,π)方向的弱色散paramagnon主导；而平面相则在（1/3,0）处出现弹性峰，磁激发无色散且能量约65 meV。偏振RIXS进一步确认了两种相中磁激发的不同特性。这些发现系统揭示了两个结构家族在基态和自旋激发上的根本差异，为理解镍酸盐超导电性的机制提供了关键信息。

材料

• Nd₉Ni₈O₂₅ (p-RP)
• Nd₉Ni₈O₁₈ (r-RP)

方法

• Resonant inelastic x-ray scattering (RIXS)
• Polarimetric RIXS
• Density functional theory (DFT) calculations
• Molecular beam epitaxy (MBE)

关键词

• spin excitations
• paramagnons
• spin density wave
• cuprate analogs
• structural families

亮点

• First comprehensive mapping of spin excitations in n=8 RP nickelates using RIXS.
• Reveals contrasting magnetic ground states between octahedral and square-planar phases.
• Provides insights for understanding nickelate superconductivity.

结论

• The octahedral p-RP develops a spin-density-wave ground state with ordering wave vector (1/4,1/4), analogous to bilayer RP.
• The square-planar r-RP shows an elastic peak at (1/3,0) and dispersionless magnetic excitations.
• The p-RP exhibits weakly dispersive paramagnons along 0→π and π→π directions, whereas r-RP displays dispersionless magnetic excitations at ≈65 meV.
• Fundamental differences in ground states and spin excitations between the two structural families are revealed.

主要论断

• The parent octahedral RP Nd₉Ni₈O₂₅ develops a spin-density-wave ground state with ordering wave vector q_SDW = (1/4, 1/4).
  • 证据: Abstract,Full text: The n=8 p-RP develops a spin-density-wave (SDW) ground state with ordering wave vector q_SDW = (1/4, 1/4).
• The reduced square-planar RP Nd₉Ni₈O₁₈ shows an elastic peak at q* = (1/3, 0) and dispersionless magnetic excitations near 65 meV.
  • 证据: Abstract,Full text: the r-RP shows an elastic peak at q* = (1/3, 0) and dispersionless magnetic excitations.
• The p-RP exhibits weakly dispersive paramagnons along the (0,π) and (π,π) directions, contrasting with the r-RP.
  • 证据: Abstract,Full text: the p-RP exhibits low-energy spectra dominated by weakly dispersive paramagnons along the (0,π) and (π,π) directions.

研究流程

• sample_preparation
  • 材料: Nd₉Ni₈O₂₅; Nd₉Ni₈O₁₈; NdGaO₃ substrate
  • 方法: ozone-assisted molecular beam epitaxy
  • 观察: high-quality thin films
• RIXS_measurement
  • 方法: Ni L₃-edge RIXS; polarimetric RIXS
  • 观察: elastic peaks; inelastic spectra
• DFT_calculation
  • 方法: WIEN₂k (APW+lo); susceptibility calculation
  • 观察: Fermi surface; density of states
• data_analysis
  • 方法: fits to damped harmonic oscillator
  • 观察: paramagnon dispersion; energy scales
• interpretation — The p-RP has SDW with q=(1/4,1/4) and weakly dispersive paramagnons; the r-RP lacks long-range order and shows dispersionless magnetic excitations.