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

摘要

本文利用功能重正化群方法，研究了双层镍酸盐La₃Ni₂O₇在常压与高压晶体结构下自旋密度波序与超导的竞争。通过对比两种结构的弱耦合多轨道模型，发现随着Hund耦合增大，主导不稳定性从超导转变为具有特征波矢Q₁≈(π/2,π/2)的自旋密度波，与实验一致。令人意外的是，常压与高压结构的非相互作用磁化率和fRG主导不稳定性几乎相同，表明压力下超导的出现不能仅由低能电子结构变化解释。进一步分析表明，抑制正交畸变是关键因素：当体系趋近四方极限时，对称性相关的自旋密度波涨落近乎简并，从而阻碍长程磁有序并增强配对相互作用。这些结果揭示了晶格对称性是调控双层镍酸盐中竞争有序态的核心参量，并提示通过单轴应变降低正交畸变有望在常压下实现体超导。

材料

• La₃Ni₂O₇ bilayer nickelate

方法

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

关键词

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

亮点

• 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.

结论

• 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.

主要论断

• The ambient- and high-pressure phases have nearly identical non-interacting susceptibilities and leading fRG instabilities.
  • 证据: 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.
  • 证据: 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.
  • 证据: 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)

研究流程

• Model Construction — The low-energy electronic structures of ambient- and high-pressure phases are remarkably similar.
  • 材料: Bilayer nickelate La₃Ni₂O₇; Ambient-pressure (Amam) structure; High-pressure (I₄/mmm) structure; Uniaxially strained structure
  • 方法: DFT band structure projection; Maximally-localized Wannier functions; Tight-binding two-orbital bilayer model
  • 观察: 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.
  • 材料: Same models as stage 1
  • 方法: Calculation of bare susceptibilities; Analysis in even and odd interlayer channels; Comparison along high-symmetry paths
  • 观察: 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.
  • 材料: Same models
  • 方法: Functional renormalization group (fRG); Truncated-unity fRG scheme; divERGe code
  • 观察: 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.
  • 材料: High-pressure model (nearly tetragonal); Ambient-pressure model with uniaxial strain (forced tetragonal-like); Ambient-pressure model (orthorhombic)
  • 方法: Comparison of RG evolution of vertex at dominant vectors; Analysis of order-parameter degeneracy
  • 观察: 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.
  • 材料: All previous results
  • 方法: Theoretical synthesis
  • 观察: Lattice symmetry is key tuning parameter