Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

摘要

利用共振X射线散射和光谱学方法，研究双层镍氧化物La₂PrNi₂O₇薄膜中自旋密度波（SDW）序与超导电性的关系。发现超导仅出现在无SDW且氧化学计量完整的区域，而氧缺失则促进SDW序，表明两者发生相分离。进一步通过Ni-L₃和O-K边光谱揭示，超导相具有金属性基态，主要呈现Ni d⁸与氧配体空穴特征；氧缺失导致电子局域化并出现低能激发。结合理论分析，提出配体空穴主要位于层间顶角氧上，形成稳定的层间五自旋极化子态，作为超导双层镍氧化物的基态。研究表明氧化学计量是控制层间耦合及电子结构的关键参数，SDW序并非超导的内在母态。

材料

• La₂PrNi₂O₇
• La₂PrNi₂O_7-x oxygen-deficient

方法

• resonant x-ray scattering
• X-ray absorption spectroscopy
• Resonant x-ray scattering
• Resonant inelastic x-ray scattering (RIXS)
• X-ray absorption spectroscopy (XAS)
• Ni-L₃ edge spectroscopy
• O-K edge spectroscopy
• DFT
• Bethe-Salpeter equation (BSE) calculations
• Spin model calculations

关键词

• spin density wave
• oxygen stoichiometry
• interlayer five spin polaron
• phase segregation
• ligand hole
• apical oxygen
• interlayer coupling
• c axis electronic structure
• small/negative charge transfer regime

亮点

• Identification of the five-spin polaron as the key electronic state for superconductivity in bilayer nickelates.
• First demonstration of SDW-free superconducting phase in compressively strained La₂PrNi₂O₇ thin films, with SDW signals confined to sparse patches near sample edges or beneath electrodes.
• Spatial mapping reveals oxygen-deficiency-driven phase segregation: SDW is uniformly distributed in oxygen-deficient films but absent in oxygen-stoichiometric superconducting regions.
• Identification of distinct localized excitations at 0.4 eV and 1.5 eV in oxygen-deficient films, absent in superconducting films, indicating enhanced localization upon oxygen loss.
• O-K edge XAS and RIXS reveal substantial ligand hole occupancy at the apical oxygen site, supporting the interlayer five-spin polaron scenario with total spin Stot = 3/2.
• The proposed five-spin polaron state provides a microscopic mechanism for ambient-pressure superconductivity in bilayer nickelates, highlighting the critical role of apical oxygen stoichiometry and interlayer coupling.

结论

• Superconductivity occurs in SDW-free, oxygen-stoichiometric regions, while oxygen deficiency promotes SDW order, indicating phase segregation.
• A ligand hole resides at the inter-bilayer apical oxygen, forming a robust interlayer five-spin polaron state that serves as the ground state for superconductivity.
• SDW order is extrinsic to superconducting bilayer nickelates and primarily driven by oxygen deficiency, which stabilizes the QSDW = (0.25, 0.25) SDW order.
• Superconductivity occurs in oxygen-stoichiometric, SDW-free regions, indicating phase segregation between SDW and superconductivity.
• Oxygen deficiency pushes the system towards a more localized, less ligand-hybridized electronic structure, while superconducting films remain more itinerant and strongly hybridized.
• The superconducting ground state involves a robust interlayer five-spin polaron formed by a ligand hole at the inter-bilayer apical oxygen, with Ni in a d8 configuration and significant oxygen ligand hole character.
• The interlayer five-spin polaron locks the out-of-plane 3dz2 Ni charge and spin configurations, leaving in-plane 3dx2-y2 orbitals near a half-filled regime, reminiscent of cuprates and infinite-layer nickelates.

主要论断

• Superconductivity occurs in SDW-free, oxygen-stoichiometric regions; oxygen deficiency promotes SDW order, indicating phase segregation.
  • 证据: Spatial mapping shows SDW signal only at sparse patches in superconducting film; uniform SDW in oxygen-deficient film
• The superconducting electronic structure is characterized by Ni d⁸ and oxygen ligand-hole character, distinct from oxygen-deficient films.
  • 证据: Ni-L₃ RIXS shows dd excitation at 1 eV and fluorescence continuum; O-K XAS shows pre-edge features indicating ligand holes
• Ligand holes primarily reside at inter-bilayer apical oxygen, forming a robust interlayer five-spin polaron state.
  • 证据: O-K XAS in c-axis polarization shows significant apical hole density; theoretical OCEAN calculations reproduce experimental XAS only with apical hole configuration
• Superconductivity occurs in SDW-free, oxygen-stoichiometric regions of bilayer nickelates, while oxygen deficiency promotes SDW order, indicating phase segregation.
  • 证据: Spatial mapping of SDW peak intensity shows SDW only in oxygen-deficient patches; superconducting regions are SDW-free.
• A ligand hole primarily occupies the inter-bilayer apical oxygen, forming an interlayer five-spin polaron (Stot=3/2) as the ground state for superconducting bilayer nickelates.
  • 证据: O-K edge XAS and RIXS reveal substantial out-of-plane ligand hole weight; DFT+BSE calculations reproduce experimental O-K pre-edge features only for the 5-spin polaron configuration.

研究流程

• sample_preparation — Thin films with controlled oxygen stoichiometry.
  • 材料: La₂PrNi₂O₇ thin films; SrLaAlO₄ substrate
  • 方法: pulsed laser deposition; ozone annealing
  • 观察: superconducting and oxygen-deficient films
• resonant_x_ray_scattering — SDW order is not intrinsic to superconducting phase.
  • 材料: Ni-L₃ edge; π polarization
  • 方法: resonant elastic x-ray scattering; spatial mapping
  • 观察: SDW peak at Q = (0.25, 0.25) localized in oxygen-deficient regions
• x_ray_spectroscopy — Superconducting phase shows metallic ground state with Ni d⁸ and oxygen ligand-hole character.
  • 材料: Ni-L₃ and O-K edges
  • 方法: XAS; RIXS
  • 观察: dd excitations at 1 eV; ligand hole character
• theoretical_modeling — Ground state of superconducting bilayer nickelates is an interlayer five-spin polaron.
  • 材料: BSE-based OCEAN code
  • 方法: DFT with SCAN functional; Bethe-Salpeter equation
  • 观察: apical oxygen hole stabilizes interlayer five-spin polaron state
• Sample Preparation — Oxygen stoichiometry is a key parameter controlling superconductivity.
  • 材料: La₂PrNi₂O₇; SrLaAlO₄ substrate
  • 方法: pulsed laser deposition; ozone annealing
  • 观察: films are coherently strained; oxygen stoichiometry controlled
• Resonant X-ray Scattering — SDW is absent in oxygen-stoichiometric superconducting regions, indicating phase segregation.
  • 材料: LPNO thin films
  • 方法: resonant x-ray scattering at Ni-L₃ edge
  • 观察: SDW peak observed at Q=(0.25,0.25) in oxygen-deficient regions; spatial mapping shows inhomogeneous SDW distribution
• X-ray Absorption and Resonant Inelastic X-ray Scattering (XAS/RIXS) — A ligand hole primarily resides at the inter-bilayer apical oxygen, forming a robust interlayer five-spin polaron state.
  • 材料: LPNO thin films
  • 方法: XAS in total fluorescence yield; RIXS at Ni-L₃ and O-K edges
  • 观察: distinct electronic structures between superconducting and oxygen-deficient films; enhanced ligand-hole character along c-axis in SC films
• BSE-OCEAN and DFT Calculations — The interlayer five-spin polaron is the ground state for superconducting LPNO.
  • 材料: LPNO
  • 方法: DFT with SCAN functional; BSE-based OCEAN code for core-level spectroscopy
  • 观察: calculated O-K XAS matches experiment for 5-spin polaron configuration