Summary
Research on the pairing mechanism of the bilayer nickelate La3Ni2O7 high-temperature superconductor demonstrates that the unified framework based on the “genesis principle” and the “synergistic Fermi surface rule” can be naturally extended to this bilayer multi-orbital system. Through strong-correlation analysis, two dominant antiferromagnetic superexchange channels are identified: the intralayer same-orbital (d_z2) nearest-neighbor exchange J⊥ mediated by the inner apical oxygen, and the interlayer different-orbital (d_z2 and dx2-y2) nearest-neighbor exchange Jxz mediated by the in-plane oxygen. Due to the bilayer bonding-antibonding splitting and the B1g symmetry of the dx2-y2 orbital, the two channels cooperate to produce a stable s± superconducting state, characterized by internal sign reversal between the mirror-even and mirror-odd Fermi surface pockets in momentum space. Both pairing channels maximize the superconducting gap on the β pocket with a form factor of (coskx − cosky)2. This result incorporates La3Ni2O7 into the unified framework of unconventional superconductivity while revealing its unique electronic environment for high-temperature superconducting pairing.
Materials
Methods
- strong-correlation analysis
- functional renormalization group (FRG) calculations
- gene principle
- collaborative Fermi surface rule analysis
Keywords
- s± superconducting state
- antiferromagnetic superexchange
- pairing form factor (coskx cosky)2
- mirror even and mirror odd fermi surface pockets
- bilayer bonding antibonding splitting
- s± pairing
- antiferromagnetic exchange
- pairing form factor
- bilayer nickelate
Highlights
- The unified framework based on the 'gene principle' and 'collaborative Fermi surface rule' is extended to the bilayer multi-orbital system.
- The two pairing channels maximize the superconducting gap on the β pocket with form factor (coskx-cosky)2.
- Places La3Ni2O7 within a unified framework for unconventional superconductivity, revealing a distinct electronic environment.
Conclusions
- The pairing symmetry of bilayer La3Ni2O7 is determined to be an s± state, with internal sign reversal between mirror-even and mirror-odd Fermi-surface pockets.
- Two dominant antiferromagnetic exchange channels cooperate: interlayer same-orbital (d_z2) exchange J⊥ and intralayer inter-orbital (d_z2 and dx2-y2) exchange Jxz.
- The gene principle and collaborative Fermi surface rule extend to the bilayer multi-orbital system, leading to an s± superconducting state with sign reversal between mirror-even and mirror-odd pockets.
Main claims
- Two dominant antiferromagnetic exchange channels exist in La3Ni2O7: interlayer same-orbital d_z2 exchange and intralayer inter-orbital d_z2-dx2-y2 exchange.
- Evidence: Through strong-correlation analysis, two dominant antiferromagnetic superexchange channels are identified: the intralayer same-orbital (d_z2) nearest-neighbor exchange J⊥ mediated by the inner apical oxygen, and the interlayer different-orbital (d_z2 and dx2-y2) nearest-neighbor exchange Jxz mediated by the in-plane oxygen.
- The two pairing channels cooperate to produce a stable s± superconducting state with internal sign reversal between mirror-even and mirror-odd Fermi surface pockets.
- Evidence: Due to the bilayer bonding-antibonding splitting and the B1g symmetry of the dx2-y2 orbital, the two channels cooperate to produce a stable s± superconducting state, characterized by internal sign reversal between the mirror-even and mirror-odd Fermi surface pockets in momentum space.
- Both pairing channels maximize the superconducting gap on the beta pocket with a form factor of (coskx - cosky)2.
- Evidence: Both pairing channels maximize the superconducting gap on the β pocket with a form factor of (coskx − cosky)2.
- Two antiferromagnetic exchange channels (interlayer intra-orbital J⊥ between dz2 orbitals and intralayer inter-orbital Jxz between dz2 and dx2-y2 orbitals) provide the dominant pairing force in bilayer La3Ni2O7.
- Evidence: Theoretical derivation based on strong-coupling perturbation theory.
- The two channels cooperate to produce a robust s± superconducting state with sign reversal between mirror-even and mirror-odd Fermi-surface pockets.
- Evidence: Mean-field gap equations show constructive addition on α and β pockets; FRG calculations confirm the s± state.
Workflow
- electronic structure and Fermi surface classification — Electronic structure shows bilayer bonding-antibonding splitting and two active Ni orbitals (d_z2 and dx2-y2).
- Materials: La3Ni2O7 compound
- Methods: density functional theory (DFT); tight-binding modeling
- Observations: identification of three Fermi pockets: alpha, beta, gamma; orbital classification by mirror symmetry and orbital phase
- identification of dominant AFM exchange interactions — Two AFM exchange channels provide the dominant pairing force.
- Materials: La3Ni2O7
- Methods: strong-correlation analysis; perturbation theory
- Observations: two dominant AFM channels: interlayer d_z2-d_z2 via apical oxygen (J⊥) and intralayer inter-orbital d_z2-dx2-y2 via in-plane oxygen (Jxz)
- pairing symmetry analysis — Pairing symmetry is s±, and the two channels cooperate to produce a robust superconducting state.
- Materials: La3Ni2O7
- Methods: strong-correlation analysis; comparison with FRG
- Observations: s± superconducting state with internal sign reversal between mirror-even and mirror-odd pockets; pairing gap maximized on beta pocket with (coskx - cosky)2 form factor
- Electronic Structure Analysis — Fermi surface pockets are classified by mirror symmetry and orbital relative phase.
- Materials: La3Ni2O7
- Methods: tight-binding model; band structure classification by mirror symmetry and orbital phase
- Observations: Three Fermi pockets: α (even, in-phase), β (odd, out-of-phase), γ (even, out-of-phase)
- Derivation of Leading AFM Exchange Interactions — Two dominant AFM exchange channels: interlayer intra-orbital and intralayer inter-orbital.
- Materials: La3Ni2O7
- Methods: strong-coupling expansion; second-order perturbation theory
- Observations: Interlayer exchange J⊥ between dz2 orbitals via apical oxygen; intralayer inter-orbital exchange Jxz between dz2 and dx2-y2 via in-plane oxygen
- Determination of Pairing Symmetry — The two channels cooperate to produce a robust s± state with internal sign reversal.
- Materials: La3Ni2O7
- Methods: mean-field decoupling of exchange interactions; projection onto band basis
- Observations: Interlayer exchange gives sign change between mirror-even and mirror-odd pockets; inter-orbital exchange selects s-wave form factor (cos kx - cos ky)2
- Comparison with FRG Calculations — Strong-coupling analysis and FRG converge to the same s± state.
- Materials: La3Ni2O7
- Methods: functional renormalization group (FRG)
- Observations: FRG also finds s± state with similar momentum space sign changes