Structural symmetry effects on the competition of density waves and superconductivity in bilayer nickelates

Summary

Using the functional renormalization group method, this study investigates the competition between spin-density wave order and superconductivity in the bilayer nickelate La₃Ni₂O₇ under both ambient and high-pressure crystal structures. B_y comparing weakly coupled multi-orbital models of the two structures, it is found that as the Hund coupling increases, the dominant instability transitions from superconductivity to a spin-density wave with a characteristic wave vector Q₁≈(π/2,π/2), consistent with experiments. Surprisingly, the non-interacting susceptibilities and fRG leading instabilities are nearly identical for the ambient and high-pressure structures, indicating that the emergence of superconductivity under pressure cannot be solely attributed to changes in low-energy electronic structure. Further analysis reveals that suppressing orthorhombic distortion is key: when the system approaches the tetragonal limit, symmetry-related spin-density wave fluctuations become nearly degenerate, thereby hindering long-range magnetic order and enhancing pairing interactions. These results highlight lattice symmetry as a crucial parameter in tuning the competing ordered states in bilayer nickelates and suggest that reducing orthorhombic distortion through uniaxial strain may enable bulk superconductivity at ambient pressure.

Materials

• La₃Ni₂O₇ bilayer nickelate

Methods

• Functional renormalization group (fRG)
• Density functional theory (DFT) calculations
• Tight-binding modeling
• Wannier projection

Keywords

• spin density wave
• superconductivity competition
• orthorhombicity
• tetragonal symmetry
• uniaxial strain

Highlights

• Ambient-pressure and high-pressure phases exhibit nearly identical non-interacting susceptibilities and leading fRG instabilities.
• The degeneracy of SDW ordering vectors in the tetragonal phase enhances magnetic fluctuations acting as pairing glue, while destabilizing long-range SDW order.

Conclusions

• The emergence of superconductivity under pressure cannot be explained solely by changes in low-energy electronic structure; instead, suppression of orthorhombicity is a key ingredient.
• As the system approaches the tetragonal limit, symmetry-related SDW fluctuations become nearly degenerate, frustrating long-range magnetic order while enhancing pairing interactions.
• Reducing orthorhombicity through uniaxial strain could stabilize bulk superconductivity at ambient pressure.

Main claims

• The ambient- and high-pressure phases have nearly identical non-interacting susceptibilities and leading fRG instabilities.
  • Evidence: Non-interacting susceptibilities show similar peaks and hierarchy (Section IV.1, Fig. 1(e-j)),fRG results show similar transition from superconductivity to SDW with increasing J_H (Section IV.1, Fig. 2)
• Suppression of orthorhombicity is a key factor for superconductivity under pressure, as it creates degenerate SDW fluctuations that enhance pairing and frustrate magnetic order.
  • Evidence: Analysis of vertex evolution shows near degeneracy of Q₁ and Q₂ in high-pressure phase (Section IV.2, Fig. 3),Strained ambient-pressure model reproduces similar degeneracy (Fig. 4)
• Applying uniaxial strain to reduce orthorhombicity could stabilize bulk superconductivity at ambient pressure.
  • Evidence: Model calculations with uniaxial strain show enhanced SDW fluctuations and pairing (Section IV.2, Fig. 4),Theoretical reasoning about Goldstone modes and frustration supports the idea (Section IV.2)

Workflow

• Model Construction — The low-energy electronic structures of ambient- and high-pressure phases are remarkably similar.
  • Materials: Bilayer nickelate La₃Ni₂O₇; Ambient-pressure (Amam) structure; High-pressure (I₄/mmm) structure; Uniaxially strained structure
  • Methods: DFT band structure projection; Maximally-localized Wannier functions; Tight-binding two-orbital bilayer model
  • Observations: Band structures and Fermi surfaces obtained; Unit cell doubling in ambient-pressure phase
• Non-interacting Susceptibility Comparison — The non-interacting susceptibilities of ambient- and high-pressure phases are nearly identical.
  • Materials: Same models as stage 1
  • Methods: Calculation of bare susceptibilities; Analysis in even and odd interlayer channels; Comparison along high-symmetry paths
  • Observations: Susceptibility peaks at Q₁ approx (pi/2, pi/2) in pseudo-tetragonal notation; Overall hierarchy of fluctuations is almost identical between phases
• fRG Calculation of Leading Instabilities — Both phases exhibit the same qualitative phase transition from superconductivity to SDW as J_H increases, but the transition occurs at higher J_H in the high-pressure phase.
  • Materials: Same models
  • Methods: Functional renormalization group (fRG); Truncated-unity fRG scheme; divERGe code
  • Observations: Low J_H: superconducting instability; High J_H: SDW instability with Q₁ approx (pi/2, pi/2); Critical J_H is higher for high-pressure phase
• Analysis of Orthorhombicity Effects — Suppressing orthorhombicity leads to near-degeneracy of SDW ordering vectors, which enhances pairing fluctuations while frustrating long-range SDW order, favoring superconductivity.
  • Materials: High-pressure model (nearly tetragonal); Ambient-pressure model with uniaxial strain (forced tetragonal-like); Ambient-pressure model (orthorhombic)
  • Methods: Comparison of RG evolution of vertex at dominant vectors; Analysis of order-parameter degeneracy
  • Observations: In high-pressure phase, Q₁ and Q₂ (symmetry-related) are nearly degenerate; in ambient pressure, Q₁ dominates; Strained model shows Q₁ and Q₂ becoming close
• Conclusion and Implications — Reducing orthorhombicity via uniaxial strain can stabilize bulk superconductivity at ambient pressure.
  • Materials: All previous results
  • Methods: Theoretical synthesis
  • Observations: Lattice symmetry is key tuning parameter