Ni-O hybridization as a stabilizer for s± superconductivity in La₃Ni₂O₇: a DFT+RPA study

Summary

This study addresses the controversy over the superconducting pairing symmetry of the high-pressure bilayer nickelate La₃Ni₂O₇ by employing a full-spectrum model based on orthogonalized projection (the Oroj method), which projects Kohn–Sham states onto local Ni-e_g orbitals and, while preserving the density functional theory band structure, redistributes spectral weight over a wider energy range, thereby incorporating Ni–O hybridization and contributions from electronic states away from the Fermi surface. Compared with a Wannier model that describes only the low-energy bands near the Fermi surface, this full-spectrum model significantly enhances interlayer spin fluctuations and yields a commensurate magnetic instability; within the spin-fluctuation framework, such features favor the formation of a sign-changing s±-wave superconducting state, whereas the low-energy model tends toward d-wave pairing. The results indicate that interlayer coupling and Ni–O hybridization in the full-energy description play an important stabilizing role in theoretical predictions of the superconducting pairing symmetry of bilayer nickelates.

Materials

• La₃Ni₂O₇ bilayer nickelate

Methods

• DFT
• orthonormalized projections (Oroj)
• matrix random-phase approximation (mRPA)
• Wannierization
• maximally localized Wannier functions
• Fermi surface nesting analysis
• RPA spin and charge susceptibility calculations

Keywords

• s± superconductivity
• d wave pairing
• spin fluctuations
• interlayer coupling
• ni o hybridization
• fermi surface nesting
• magnetic instability
• pairing symmetry

Highlights

• The Oroj method uses the complete set of Kohn-Sham bands as the non-interacting basis, reducing the many-band problem to a few-orbital model without altering the DFT eigenvalues.
• The full-spectrum treatment reveals strong interlayer spin fluctuations that are absent or underestimated in low-energy Wannier models.
• The predicted s± pairing from the full-spectrum weak-coupling treatment aligns with strong-coupling results, helping resolve discrepancies among low-energy tight-binding models.
• The model attributes the stabilization of s± pairing to an extra electron arising from hybridization with oxygen states far from the Fermi level.

Conclusions

• The orthonormalized-projection full-spectrum model preserves the density-functional band structure while redistributing spectral weight over a wide energy range, naturally including Ni-O hybridization and states away from the Fermi level.
• Compared with the low-energy Wannier model, the full-spectrum Oroj description yields enhanced interlayer spin fluctuations and a commensurate magnetic instability.
• Within the spin-fluctuation framework, the full-spectrum features favor a sign-changing s± superconducting state, whereas low-energy models tend to stabilize d-wave pairing.
• The Oroj electron occupation is consistent with a Ni-O molecular-orbital picture with five holes per Ni-O dimer, and the additional hybridized electron stabilizes interlayer pairing.
• Interlayer coupling in full-energy models plays an important role in shaping the predicted pairing symmetry of bilayer nickelates.

Main claims

• The full-spectrum Oroj model includes Ni-O hybridization and high-energy spectral weight, changing the effective Ni orbital occupation relative to low-energy Wannier models.
  • Evidence: The Oroj construction preserves the DFT band structure while redistributing spectral weight over a wide energy range.,Oroj naturally includes contributions from Ni-O hybridized bands located well below/above the Fermi level, adding spectral weight and increasing overall Ni orbital occupation.,Wannier models restricted to low-energy bands yield lower orbital occupations and omit oxygen-derived weight.
• The Oroj full-spectrum model gives enhanced interlayer spin fluctuations and a commensurate magnetic instability.
  • Evidence: Compared to a low-energy description, this approach yields enhanced interlayer spin fluctuations and a commensurate magnetic instability.,Oroj susceptibility matrix shows diverging spin-density terms plus well-pronounced interlayer terms; Wannier model shows irrelevant interlayer terms.,The Oroj magnetic order parameter includes interlayer spin-bond order absent in the Wannier model.
• Within the spin-fluctuation framework, the full-spectrum Oroj model favors sign-changing s± superconductivity, whereas low-energy models favor d-wave pairing.
  • Evidence: Within a spin-fluctuation framework, these features favor a sign-changing s± superconducting state, whereas low-energy models tend to stabilize d-wave pairing.,Fig. 9 shows robust s± symmetry for the Oroj model and d-wave symmetry for the Wannier model.,The Oroj solution aligns with strong-coupling results predicting s±-wave superconductivity.
• Interlayer coupling and apical-oxygen-mediated hybridization play an important stabilizing role in theoretical predictions of pairing symmetry in bilayer nickelates.
  • Evidence: Interlayer coupling in full-energy models may play an important role in shaping the predicted pairing symmetry.,The interlayer fluctuations are highly influenced by the apical oxygen site that connects dz2 orbitals in both layers.,In a simplified bilayer molecular-orbital model, bonding/antibonding gaps directly relate interlayer and intralayer pairing components, and interlayer pairing is essential for s±.
• Truncated Wannier-based weak-coupling models can produce fragile d-wave pairing due to distorted Fermi-surface nesting and sensitivity to perturbations.
  • Evidence: The Wannier Fermi surface shows smaller and slightly distorted pockets, leading to incorrect magnetic nesting vector.,In Wannier-based tight-binding models, superconducting pairing is highly sensitive to the range of hopping amplitudes.,Emergence of d-wave superconductivity in truncated Wannier models is likely accidental and dependent on fine-tuned band structures.

Workflow

• model_construction — Full-spectrum projected models capture Ni-O hybridization and high-energy spectral weight absent in low-energy Wannier models.
  • Materials: La₃Ni₂O₇ high-pressure bilayer nickelate; Ni e_g orbitals (dx2-y2, dz2); oxygen hybridized states
  • Methods: density functional theory Kohn-Sham eigenstates; orthonormalized projections (Oroj) onto local orbitals; maximally localized Wannier functions for low-energy comparison
  • Observations: Oroj preserves the DFT band structure and redistributes spectral weight over a wide energy range; Wannier model reproduces only four low-energy bands and distorts Fermi surface pockets; Oroj yields larger projected Ni orbital occupation, corresponding to an extra electron compared with Wannier
• electronic_structure_analysis — The full-spectrum Oroj approach gives a more faithful Fermiology and electron occupation than truncated low-energy Wannier models.
  • Materials: La₃Ni₂O₇ Fermi surface; Oroj spectral weights; Wannier band structure
  • Methods: band structure and Fermi surface comparison; orbital-resolved spectral function and density of states; integrated electron occupation analysis
  • Observations: Oroj and DFT Fermi surfaces show nesting between the nearest alpha and beta pockets; Wannier Fermi surface has smaller, distorted pockets and incorrect nesting vector; Oroj orbital occupations for dx2-y2 and dz2 are similar, unlike Wannier values
• magnetic_susceptibility_analysis — The Oroj full-spectrum model enhances interlayer spin fluctuations and produces a commensurate magnetic instability.
  • Materials: interacting multiorbital model with Hubbard and Hund interactions; Oroj and Wannier non-interacting bases
  • Methods: matrix random-phase approximation (mRPA); bare and interacting spin susceptibility calculations; generalized Stoner criterion and susceptibility eigen-decomposition
  • Observations: Oroj has a susceptibility peak at a commensurate magnetic ordering vector; Wannier model peaks at incommensurate vector and lacks strong interlayer susceptibility terms; Oroj order parameter includes interlayer spin-bond order; Wannier does not
• superconducting_pairing_analysis — Ni-O hybridization and interlayer coupling stabilize s± superconductivity in La₃Ni₂O₇, whereas low-energy models favor d-wave.
  • Materials: Oroj and Wannier interacting models; spin-fluctuation pairing interaction
  • Methods: multiorbital RPA pairing interaction; linearized gap equation solved using spectral-weight formulation
  • Observations: Oroj yields a sign-changing s± gap; Wannier model yields d-wave pairing; Oroj s± solution is connected to interlayer pairing and remains robust