Interlayer Five-Spin Polaron in Superconducting Bilayer Nickelates

Summary

Using resonant X-ray scattering and spectroscopy, we investigate the relationship between spin-density wave (SDW) order and superconductivity in bilayer nickelate La₂PrNi₂O₇ thin films. Superconductivity is found to emerge only in regions without SDW and with complete oxygen stoichiometry, whereas oxygen deficiency promotes SDW order, indicating phase separation between the two. Further Ni-L₃ and O-K edge spectroscopy reveal that the superconducting phase exhibits a metallic ground state dominated by Ni d⁸ and oxygen ligand-hole character; oxygen deficiency induces electron localization and the appearance of low-energy excitations. Combined with theoretical analysis, we propose that ligand holes primarily reside on the interlayer apical oxygen sites, forming stable interlayer five-spin polaron states as the ground state of superconducting bilayer nickelates. This study demonstrates that oxygen stoichiometry is a key parameter controlling interlayer coupling and electronic structure, and that SDW order is not the intrinsic parent state of superconductivity.

Materials

• La₂PrNi₂O₇
• La₂PrNi₂O_7-x oxygen-deficient

Methods

• 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

Keywords

• 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

Highlights

• 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.

Conclusions

• 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.

Main claims

• Superconductivity occurs in SDW-free, oxygen-stoichiometric regions; oxygen deficiency promotes SDW order, indicating phase segregation.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.

Workflow

• sample_preparation — Thin films with controlled oxygen stoichiometry.
  • Materials: La₂PrNi₂O₇ thin films; SrLaAlO₄ substrate
  • Methods: pulsed laser deposition; ozone annealing
  • Observations: superconducting and oxygen-deficient films
• resonant_x_ray_scattering — SDW order is not intrinsic to superconducting phase.
  • Materials: Ni-L₃ edge; π polarization
  • Methods: resonant elastic x-ray scattering; spatial mapping
  • Observations: 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.
  • Materials: Ni-L₃ and O-K edges
  • Methods: XAS; RIXS
  • Observations: dd excitations at 1 eV; ligand hole character
• theoretical_modeling — Ground state of superconducting bilayer nickelates is an interlayer five-spin polaron.
  • Materials: BSE-based OCEAN code
  • Methods: DFT with SCAN functional; Bethe-Salpeter equation
  • Observations: apical oxygen hole stabilizes interlayer five-spin polaron state
• Sample Preparation — Oxygen stoichiometry is a key parameter controlling superconductivity.
  • Materials: La₂PrNi₂O₇; SrLaAlO₄ substrate
  • Methods: pulsed laser deposition; ozone annealing
  • Observations: films are coherently strained; oxygen stoichiometry controlled
• Resonant X-ray Scattering — SDW is absent in oxygen-stoichiometric superconducting regions, indicating phase segregation.
  • Materials: LPNO thin films
  • Methods: resonant x-ray scattering at Ni-L₃ edge
  • Observations: 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.
  • Materials: LPNO thin films
  • Methods: XAS in total fluorescence yield; RIXS at Ni-L₃ and O-K edges
  • Observations: 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.
  • Materials: LPNO
  • Methods: DFT with SCAN functional; BSE-based OCEAN code for core-level spectroscopy
  • Observations: calculated O-K XAS matches experiment for 5-spin polaron configuration