Electronic and magnetic excitations in La₃Ni₂O₇

摘要

High-temperature superconductivity was discovered in the pressurized nickelate La₃Ni₂O₇ which has a unique bilayer structure and mixed valence state of nickel. The properties at ambient pressure contain crucial information of the fundamental interactions and bosons mediating superconducting pairing. Here, using X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we identified that Ni 3d_x2-y2, Ni 3d_z2, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy. Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing the strong electronic correlation and rich magnetic properties. Based on an effective Heisenberg spin model, we extract a much stronger inter-layer effective magnetic superexchange than the intra-layer ones and propose two viable magnetic structures. Our findings emphasize that the Ni 3d_z2orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La₃Ni₂O₇ superconductor.

材料

• La₃Ni₂O₇

方法

• X-ray absorption spectroscopy (XAS)
• resonant inelastic X-ray scattering (RIXS)

关键词

• charge transfer energy
• spin density wave ordering
• magnetic superexchange
• orbital bonding

亮点

• Our findings emphasize that the Ni 3dz2 orbital bonding within the bilayer induces novel electronic and magnetic excitations, setting the stage for further exploration of La₃Ni₂O₇ superconductor.

结论

• Ni 3dx2-y2, Ni 3dz2, and ligand oxygen 2p orbitals dominate the low-energy physics with a small charge-transfer energy.
• Well-defined optical-like magnetic excitations soften into quasi-static spin-density-wave ordering, evidencing strong electronic correlation and rich magnetic properties.
• Based on an effective Heisenberg spin model, a much stronger inter-layer effective magnetic superexchange than the intra-layer ones is extracted.
• Two viable magnetic structures are proposed.

主要论断

• The electronic structure is dominated byNi 3dx2-y2, Ni 3d_z2, and ligand oxygen 2p orbitals with a small charge-transfer energy (<2 eV).
  • 证据: XAS pre-edge peak at O K-edge indicates ligand holes,Multiplet calculations reproduce XAS and RIXS with Δ=0.5 eV
• Well-defined dispersive optical-like magnetic excitations soften into a quasi-static spin-density-wave order at (0.25,0.25), evidencing strong electronic correlation.
  • 证据: RIXS shows magnon dispersion to ≈70 meV that softens to zero at (0.25,0.25),Polarimetric RIXS confirms magnetic origin,SDW peak at 0.25 r.l.u. disappears above 150K
• Inter-layer effective magnetic superexchange (JzS) is much stronger (≈67–74 meV) than intra-layer ones.
  • 证据: Fitting magnon dispersion with Heisenberg model gives JzS ≈ 67–74 meV, J₁S ≈ 0–3.6 meV, J₂S ≈ 2–4 meV

研究流程

• sample_fabrication — High-quality single crystals obtained.
  • 材料: La₃Ni₂O₇ single crystals
  • 方法: high oxygen pressure floating zone technique
  • 观察: Sample quality checked by XRD and Laue diffraction
• xas_and_rixs_measurements — Strong electronic correlations and magnetic excitations are observed.
  • 材料: La₃Ni₂O₇ single crystal
  • 方法: X-ray absorption spectroscopy (XAS); Resonant inelastic X-ray scattering (RIXS) at Ni L₃-edge and O K-edge; Polarimetric RIXS
  • 观察: XAS shows strong pre-edge peak at O K-edge; Ni L₃-XAS shows sharp resonance at 852.4 eV; RIXS reveals dispersive low-energy excitations (≈70 meV max) that soften at (0.25,0.25); Polarimetric RIXS confirms magnetic origin
• data_analysis_and_modeling — Ni 3dx2-y2, 3d_z2, and O 2p orbitals dominate low-energy physics; inter-layer magnetic coupling is dominant.
  • 材料: XAS and RIXS spectra
  • 方法: Multiplet calculations on double-cluster model; Effective J₁-J₂-Jz Heisenberg model fitted to magnon dispersion; DFT+U calculations
  • 观察: Charge-transfer energy Δ < 2 eV; Inter-layer superexchange JzS ≈ 67–74 meV, much larger than intra-layer; SDW order at Q = (0.25, 0.25) with correlation length 27.7 nm