Spin and orbital excitations in undoped infinite layers: a comparison between superconducting PrNiO₂ and insulating CaCuO₂

摘要

该研究通过动量分辨与偏振分辨共振非弹性X射线散射（RIXS）测量，系统比较了未掺杂超导无限层镍酸盐PrNiO₂与绝缘铜酸盐CaCuO₂的自旋和轨道激发特性。结果显示，PrNiO₂的面内磁交换积分（约46 meV）明显小于CaCuO₂（约82 meV），而面外交换积分相近（约6-7 meV），表明两种材料均支持三维反铁磁有序，且自旋-自旋关联的三维性类似。轨道激发（3d态内跃迁）在单离子模型下吻合良好，但Ni-dxy峰的能量显著低于Cu-dxy，且色散方向相反——镍酸盐中呈现最近邻轨道超交换耦合驱动的轨道激发传播，而铜酸盐则为次近邻耦合主导。尽管两者电荷转移能差异显著（镍酸盐更大），但自旋与轨道激发特性总体上高度相似，仅在Ni-dxy峰能量和色散上表现出关键区别，这归因于轨道超交换耦合机制的不同。该工作揭示了无限层镍酸盐与铜酸盐在磁性和轨道动力学上的核心共性，同时指出镍酸盐中自旋涨落能量更小、掺杂电荷在金属位点上的局域化更强。

材料

• PrNiO₂
• CaCuO₂

方法

• Resonant Inelastic X-ray Scattering (RIXS)
• Polarization-resolved RIXS
• Linear Spin Wave (LSW) theory
• Single-ion cross-section calculations
• Spin-orbital model

关键词

• three dimensional antiferromagnetic order
• orbital superexchange coupling
• charge transfer energy
• self doping
• orbiton propagation

亮点

• Momentum- and polarization-resolved RIXS measurements on nominally undoped, superconducting PrNiO₂ are compared with the reference infinite layer cuprate CaCuO₂.
• The Ni-dxy peak lies at significantly lower energy and shows an opposite dispersion to that of Cu-dxy, attributed to different orbital superexchange couplings.
• The in-plane exchange integrals are approximately half in PrNiO₂ (46 meV) compared to CaCuO₂ (82 meV), while out-of-plane values are comparable (6-7 meV).
• In PrNiO₂, the magnon peak broadening is nearly constant (≈30 meV) over the explored momentum range, less than half the broadening in superconducting Bi₂₂₀₁ at lowest doping.

结论

• In PrNiO₂, the in-plane magnetic exchange integrals are smaller than in CaCuO₂, whereas the out-of-plane values are similar, indicating that both materials support a three-dimensional antiferromagnetic order.
• The orbital dispersion in the infinite-layer nickelate primarily involves nearest neighbor orbital superexchange interaction, which in cuprates is strongly hampered by coupling to magnons.
• Infinite-layer nickelates are unconventional superconductors closely related to cuprates, but lacking some of the ingredients that enhance T_c in the latter.
• Self-doping has a much milder impact on spin order than chemical doping, endowing infinite-layer nickelates with a non-disruptive way to achieve superconductivity which is absent in copper oxides.

主要论断

• In-plane magnetic exchange integral in PrNiO₂ (≈46 meV) is about half that in CaCuO₂ (≈82 meV), but out-of-plane exchange is similar (≈6-7 meV), indicating comparable three-dimensional antiferromagnetic correlations.
  • 证据: LSW fits to RIXS spin wave dispersions give J₁ values; out-of-plane exchange Jc is comparable
• Orbital excitations show distinct differences: Ni-dxy peak is lower in energy (1.29 eV) than Cu-dxy (1.65 eV) and exhibits opposite dispersion direction.
  • 证据: RIXS maps show dispersion of dxy peak; PNO dispersion is maximal at Γ and decreases; CCO is minimal at Γ and increases
• The opposite orbiton dispersion is due to dominant nearest-neighbor orbital superexchange in PNO versus next-nearest-neighbor in CCO, originating from different covalency and charge-transfer energy.
  • 证据: Three-band model calculations give NN orbital exchange dominant in PNO; NNN dominant in CCO; NN orbiton hopping in PNO enabled by Hund's exchange

研究流程

• sample_preparation — High-quality infinite-layer thin films of both systems.
  • 材料: PrNiO₂ thin films; CaCuO₂ thin films
  • 方法: pulsed laser deposition; topotactic reduction for PrNiO₂
  • 观察: PrNiO₂ is superconducting (T_c ≈10 K); CaCuO₂ is insulating
• rixs_measurements — Comparative spin and orbital excitation spectra.
  • 材料: Ni-L₃ edge for PrNiO₂; Cu-L₃ edge for CaCuO₂
  • 方法: momentum-resolved RIXS with polarization analysis; ERIXS spectrometer at ESRF ID₃₂
  • 观察: spin excitations up to ≈230 meV (PNO) and ≈320 meV (CCO); orbital excitations at 1-3 eV
• spin_wave_analysis — In-plane exchange smaller in nickelate; out-of-plane similar.
  • 材料: RIXS dispersion data
  • 方法: linear spin wave theory with SpinW
  • 观察: J₁ ≈46 meV (PNO) vs 82 meV (CCO); Jc ≈6 meV (PNO) vs 7 meV (CCO)
• orbital_excitation_analysis — Orbiton propagation driven by nearest-neighbor orbital superexchange in PNO vs next-nearest-neighbor in CCO.
  • 材料: RIXS orbital excitation data
  • 方法: single-ion cross-section calculations; orbiton dispersion fitting
  • 观察: dxy peak at 1.29 eV (PNO) vs 1.65 eV (CCO); opposite dispersion direction