Magnetic Order in bilayer Ruddlesden-Popper Nickelates

摘要

基于轨道选择性电子关联的实验证据，本文针对双层镍酸盐La₃Ni₂O₇提出了一种新的磁性描述框架。在正常态的坏金属区中，体系邻近轨道选择性Mott相，电子谱分解为相干的d_(x²-y²)轨道准粒子和非相干的d_(z²)轨道局域磁矩。通过整合局域磁矩间的超交换作用与相干电子介导的RKKY相互作用，构建有效自旋模型，其中RKKY贡献主导了第三近邻耦合并引发磁阻挫。该模型自然稳定了一个波矢接近(π/2, π/2)的非共面反铁磁基态，层间呈反铁磁堆叠，与中子散射等实验一致。计算的自旋波谱包含低频声学支和高频光学支，声学支在序波矢处软化，总带宽约80 meV，与共振非弹性X射线散射实验相符。这一结果揭示轨道选择性关联是决定双层镍酸盐磁性的核心要素，并指出低能磁涨落及短程交换作用可能为非常规超导配对提供驱动力。

材料

• La₃Ni₂O₇

方法

• Slave-spin approach
• Luttinger–Tisza method
• Variational optimization
• Spin-wave theory
• Bilayer two-orbital Hubbard model

关键词

• orbital selective correlations
• rkky interaction
• superexchange
• incommensurate antiferromagnetism
• spin wave spectrum
• high temperature superconductivity

亮点

• The magnetic order is described in a conceptually new way via a combination of RKKY and superexchange interactions among effective local moments.
• The model yields an incommensurate antiferromagnetic ground state with interlayer antiferromagnetic stacking and wave vector near (π/2,π/2) without fine-tuning.
• The spin-wave spectrum reveals intense low-energy fluctuations and optical modes, suggesting these magnetic correlations may drive superconducting pairing.

结论

• A theoretical framework based on orbital-selective electronic correlations is developed to understand the magnetic properties of La₃Ni₂O₇.
• The effective spin Hamiltonian, comprising RKKY and superexchange couplings, stabilizes an incommensurate antiferromagnetic order with wave vector near (π/2,π/2).
• The calculated spin-wave spectrum shows an acoustic branch softening at the ordering wave vector and an overall bandwidth of about 80 meV, consistent with RIXS and neutron scattering experiments.
• Orbital-selective correlations are essential for the magnetism in bilayer nickelates.

主要论断

• Orbital-selective correlations in the bad-metal regime give rise to a combination of RKKY and superexchange interactions that determine the magnetic properties of La₃Ni₂O₇.
  • 证据: Abstract: 'Based on the accumulated experimental evidence for a bad metal state in proximity to an orbital-selective Mott phase, here we describe magnetic correlations … in terms of effective local moments experiencing a combination of RKKY and superexchange interactions.',Models section: 'the spectral weight of the d_(x²-y²) orbital resides more in the coherent part, while that of the d_(z²) orbital mainly populates the incoherent part' and 'Two distinct processes contribute to the exchange coupling … superexchange … and RKKY coupling.'
• The effective spin model stabilizes an incommensurate antiferromagnetic order with wavevector near (π/2,π/2), consistent with neutron scattering.
  • 证据: Magnetic order section: 'the ground state is an antiferromagnet … magnetic order is stabilized at the wave vector Q. At U=3 eV, Q≈(0.24,0.24) … close to (π/2,π/2)',Ibid: 'consistent with the magnetic order observed experimentally' and references to neutron diffraction (Refs. 57–59).
• The calculated spin-wave spectrum has acoustic branch softening at Q and an overall bandwidth of ≈80 meV, matching RIXS and inelastic neutron scattering data.
  • 证据: Magnetic order and excitations section: 'the calculated spin-wave spectrum … comprises a low-energy acoustic branch and a higher-energy optical branch; the acoustic branch softens at the ordering wave vector Q … overall bandwidth of the calculated spectrum is approximately 80 meV, in good agreement with the energy scale observed in recent RIXS experiments [73].'
• Orbital-selective correlations are an essential ingredient for magnetism in bilayer nickelates and may provide the pairing glue for superconductivity.
  • 证据: Introduction: 'Our results underscore the essential role of orbital-selective electron correlations in determining the magnetic properties of the bilayer nickelate and provide direct insight into understanding the origin of high-temperature superconductivity',Discussions: 'the relatively short-range exchange interactions can drive superconducting pairing … tuning a non-thermal parameter … weakens the long-range magnetic order and allows the same electrons to instead develop superconducting pairing.'

研究流程

• orbital-selective renormalization — Orbital-selective correlations justify low-energy description: local moments from d_z2 coupled to itinerant dx2-y2 electrons.
  • 材料: bilayer two-orbital Hubbard model; DFT-based tight-binding parameters; slave-spin mean-field approach
  • 方法: renormalization of hopping and onsite energies using slave-spin; separation into coherent (dx2-y2) and incoherent (d_z2) sectors
  • 观察: dx2-y2 orbital mostly coherent; d_z2 orbital largely incoherent, forming local moments
• exchange interaction derivation — Frustrated RKKY interactions are the dominant intralayer couplings; combined with superexchange they define the magnetic Hamiltonian.
  • 材料: renormalized quasiparticle Hamiltonian; static susceptibility formula; inter-orbital exchange coupling J_cf
  • 方法: RKKY interaction via susceptibility and Fourier transform; superexchange estimate from slave-spin weights
  • 观察: third-nearest-neighbor RKKY coupling J₃ dominates (J₃/J₁ > 1); interlayer superexchange J_perp ≈ 23 meV
• magnetic ground state analysis — The model naturally stabilizes an antiferromagnetic order with wavevector near (π/2,π/2), consistent with neutron diffraction.
  • 材料: effective J₁-J₃ bilayer spin model; Luttinger-Tisza method; variational optimization
  • 方法: minimization of classical magnetic energy; wavevector refinement
  • 观察: noncoplanar antiferromagnetic ground state; ordering wavevector Q near (π/2,π/2); gradual shift of Q with correlation strength
• spin-wave dispersion calculation — The calculated spin-wave spectrum matches RIXS and inelastic neutron scattering data, confirming the magnetic model.
  • 材料: optimized classical ground state; linear spin-wave theory
  • 方法: dispersion calculation with incommensurate order and zone folding
  • 观察: acoustic and optical branches; acoustic mode softening at Q; overall bandwidth ≈80 meV