Interlayer hybridization enables superconductivity in bilayer nickelates

Summary

B_y stabilizing bilayer nickelate (La,Pr)₃Ni₂O₇ superconducting thin films with a protective capping layer and employing X-ray absorption and resonant inelastic X-ray scattering spectroscopy, this study directly probes the evolution of electronic structures across insulating, superconducting, and metallic states. Experimental and theoretical analyses reveal that the in-plane dx²-y² states constitute an itinerant electron backbone, whereas superconductivity emerges only when the out-of-plane dz²-p_z-dz² interlayer hybridization becomes coherent, accompanied by suppression of static spin order and the appearance of strongly damped spin excitations. Oxygen stoichiometry and epitaxial strain jointly regulate this interlayer channel, confining the superconducting phase to a narrow window of interlayer coherence and correlation strength. These findings elucidate the microscopic prerequisites for superconductivity in bilayer nickelates and provide a multi-orbital framework to describe its emergent mechanism.

Materials

• (La,Pr)₃Ni₂O₇

Methods

• X-ray absorption spectroscopy (XAS)
• resonant inelastic X-ray scattering (RIXS)
• density functional theory (DFT) calculations
• ligand-field multiplet theory
• X-ray absorption spectroscopy
• resonant inelastic X-ray scattering
• theoretical analysis

Keywords

• interlayer hybridization
• oxygen stoichiometry
• spin density wave
• orbital selective
• spin density waves
• itinerant states
• multiorbital physics

Highlights

• The in-plane dx2-y2 states form an itinerant electron backbone.
• The study provides a multiorbital framework to describe the emergent mechanism of superconductivity in bilayer nickelates.
• Overcome limitation by stabilizing superconducting (La,Pr)₃Ni₂O₇ thin films with a protective capping layer, enabling direct spectroscopic access via X-ray absorption and resonant inelastic X-ray scattering.

Conclusions

• Superconductivity emerges only when coherent d_z2-p_z-d_z2 interlayer hybridization develops, accompanied by suppression of static spin order and appearance of strongly damped spin excitations.
• Oxygen stoichiometry and epitaxial strain jointly regulate the interlayer channel, confining the superconducting phase to a narrow window.
• Superconductivity emerges only when coherent dz2-p_z-dz2 interlayer hybridization develops, accompanied by suppressed static spin order and strongly damped spin excitations.
• Oxygen stoichiometry and epitaxial strain both act on this interlayer channel, placing superconductivity within a narrow window of interlayer coherence and correlation strength.

Main claims

• Superconductivity in bilayer nickelates emerges only when coherent interlayer dz2-pz-dz2 hybridization develops, accompanied by suppression of static spin order
  • Evidence: XAS shows out-of-plane spectral weight increases; RIXS shows SDW peak strongly suppressed in superconducting samples; calculated XAS confirms oxygen vacancies disrupt hybridization
• The evolution from insulating to superconducting to metallic states in LaPr₂Ni₂O₇ is dominated by changes in the out-of-plane electronic structure, specifically the activation of coherent dz2-pz-dz2 interlayer hybridization.
  • Evidence: O K-edge XAS shows spectral weight transfer in out-of-plane component; Ni L₃-edge RIXS shows broadening of orbital excitations; SDW is suppressed.
• Superconductivity emerges only within a narrow window of oxygen stoichiometry, where static SDW order is suppressed while robust spin exchange interactions persist.
  • Evidence: Insulating sample has inner apical oxygen vacancies and strong SDW; SC sample has optimal oxygen; metallic sample has interstitial oxygen and is overdoped.
• Oxygen stoichiometry acts as a dual tuning parameter controlling both carrier density and correlation strength.
  • Evidence: Vacancies induce localization and magnetism; interstitials drive overdoped metallic state; optimal oxygen yields coherent interlayer hybridization and superconductivity.

Workflow

• Thin film growth and capping — Capping layer preserves superconductivity for spectroscopy
  • Materials: (La,Pr)₃Ni₂O₇ films on SLAO with amorphous PBCO cap
  • Methods: PLD; E_x situ capping
  • Observations: Transport confirms superconductivity preserved
• X-ray absorption and RIXS measurements — Interlayer dz2-pz-dz2 hybridization is key for superconductivity
  • Materials: XAS at O K-edge and Ni L-edge; RIXS at Ni L-edge
  • Methods: Polarization-dependent XAS; RIXS with 85 meV resolution
  • Observations: In-plane and out-of-plane spectral evolution; SDW peak suppression; Orbital excitation broadening
• Thin Film Fabrication and Capping — Protective capping layer preserves superconductivity during XAS/RIXS measurements.
  • Materials: LaPr₂Ni₂O₇; SrLaAlO₄ substrate; amorphous PrBa₂Cu₃O₇ capping
  • Methods: pulsed laser deposition (PLD); ex situ capping
  • Observations: films with thickness 7-14 nm; insulating, superconducting, and metallic samples
• X-ray Absorption Spectroscopy (XAS) — O K-edge XAS reveals progressive hole doping and reconstruction of dz2-derived states.
  • Materials: LaPr₂Ni₂O₇ films with PBCO cap
  • Methods: O K-edge and Ni L₃-edge XAS in TFY and TEY modes
  • Observations: In-plane XAS shows systematic peak shift to lower energy from insulator to superconductor to metal; out-of-plane XAS shows two components, with spectral weight transfer
• Resonant Inelastic X-ray Scattering (RIXS) — Enhanced itinerancy in SC and metallic samples; magnon damping increases.
  • Materials: LaPr₂Ni₂O₇ films
  • Methods: RIXS at Ni L₃ edge with π polarization; scattering angles 90° and 150°
  • Observations: Orbital excitations at 0.4, 1.0, 1.5 eV broaden in SC and metallic samples; magnetic excitation at ≈70 meV broadens
• Spin Density Wave (SDW) Measurements — SDW order is suppressed in the superconducting state.
  • Materials: LaPr₂Ni₂O₇ films
  • Methods: quasi-elastic RIXS intensity along (H,H) direction
  • Observations: Pronounced SDW peak near (0.25,0.25) in insulating sample; strongly suppressed in SC sample
• DFT Calculations of O K-edge XAS — Inner apical oxygen vacancies in insulating sample disrupt interlayer hybridization.
  • Materials: Pr₃Ni₂O₇, Pr₃Ni₂O₆.75, Pr₃Ni₂O₇.25
  • Methods: VASP with PBE+U (U=4 eV on Ni); core-hole approach
  • Observations: Calculated spectra reproduce overall energy shifts; high-energy out-of-plane feature missing in DFT