Tunable Superconductivity in 1313-La₃Ni₂O₇: Suppressed under Compression and Possible s± Pairing under Tension

摘要

通过结合密度泛函理论和随机相位近似，系统研究了压缩和拉伸应变对1313-La₃Ni₂O₇薄膜超导电性的影响。研究发现，无论压应变还是拉应变，单层与三层块体间均存在自掺杂效应，且在拉伸应变下最为显著。在LSAO衬底引入的压缩应变下，即使考虑衬底锶离子迁移带来的空穴掺杂，超导电性也难以出现，这与实验一致。然而，在KTO衬底施加的拉伸应变下，三层子系统中原本未穿过费米能级的能带下移，在M点出现一个小型空穴型γ口袋，与Γ点的小型电子型σ口袋通过近(π,π)波矢相连。随机相位近似计算揭示，此时三层子系统可形成稳定的s±波配对态，其序参量在这两个口袋间发生符号反转。进一步分析表明，γ口袋大小对配对至关重要，过大的γ口袋会抑制超导。该工作预测了应变驱动的电子结构重构，并提出通过拉伸应变工程可在环境压力下实现1313-La₃Ni₂O₇超导电性的设计原则。

材料

• 1313-La₃Ni₂O₇

方法

• DFT
• random phase approximation (RPA)
• tight-binding model
• maximally localized Wannier functions

关键词

• s± wave pairing
• strain tunability
• self doping
• fermi surface nesting
• spin density wave
• charge transfer
• superconductivity

亮点

• Under tensile strain, a robust s±-wave pairing emerges with sign changes between the electron-like σ pocket at Γ and the hole-like γ pocket at M, connected by a near-(π,π) wavevector.
• The self-doping effect between single-layer and trilayer blocks is most pronounced under tensile strain, enhancing electron transfer to the trilayer.
• Superconductivity can be engineered in 1313-La₃Ni₂O₇ thin films using tensile-strain substrates like KTO, offering a route to ambient-pressure superconductivity.
• The pairing is sensitive to the size of the γ pocket, requiring an optimal size to avoid suppression.

结论

• The single-layer subsystem exhibits an instability driven by near-perfect nesting of Fermi surface sheets.
• Strain-tunable superconductivity is found: compressive strain on LSAO suppresses superconductivity even with Sr migration, while tensile strain on KTO induces a leading s±-wave pairing state in the trilayer subsystem.
• Hole doping weakens the pairing instability, while electron doping boosts pairing in 1313-LNO on KTO.
• Superconductivity requires an optimally sized γ pocket; an oversized γ pocket suppresses pairing.
• These results provide design guidelines for realizing ambient-pressure superconductivity in 1313-LNO films via tensile strain engineering.

主要论断

• Under compressive strain (LSAO substrate), superconductivity is unlikely in 1313-LNO films, even with hole doping from Sr migration.
  • 证据: RPA calculations show weak s± pairing eigenvalue λ ≈0.05 and preemption by SL SDW instability.,Hole doping further suppresses the already small γ pocket and pairing correlations.
• Under tensile strain (KTO substrate), a robust s±-wave pairing state emerges in the trilayer subsystem due to nesting between the electron-like σ pocket at Γ and the hole-like γ pocket at M.
  • 证据: DFT+ RPA reveal a leading s± eigenvalue λ=0.23 with sign change between σ and γ pockets.,Spin susceptibility shows strong AFM peak at M, with outer-layer antiferromagnetic coupling.
• Superconductivity in 1313-LNO requires an optimally sized γ pocket; electron doping boosts pairing while hole doping suppresses it, and an oversized γ pocket eliminates superconductivity.
  • 证据: RPA doping scans: λ decreases with hole doping, increases with electron doping up to ≈0.03, then drops sharply as γ pocket vanishes.,Pairing strength is maximized when γ pocket is small but finite.

研究流程

• Model Construction — Under compressive strain, the electronic structure lacks the γ pocket, and charge self-doping redistributes electrons between SL and TL blocks.
  • 材料: 1313-La₃Ni₂O₇ thin film; LSAO substrate; DFT code (VASP); WANNIER₉₀
  • 方法: Density functional theory (DFT); Maximally localized Wannier functions; Tight-binding model (8-band Ni-eg orbitals)
  • 观察: Self-doping: electron transfer from single-layer (SL) to trilayer (TL) block (occupancies 1.81 SL, 4.19 TL); TL bonding band does not cross the Fermi level → no γ pocket at M point; SL subsystem shows nested Fermi surface sheets susceptible to SDW
• Pairing Instability Analysis under Compression — Superconductivity is unlikely in 1313-LNO on LSAO, even with Sr migration, because pairing correlations are weak and competing magnetic order dominates.
  • 方法: Multi-orbital random-phase approximation (RPA); Restricted TL-only model (SL Coulomb elements set to zero); Spin susceptibility and pairing eigenvalue equation
  • 观察: Leading s±-wave pairing eigenvalue λ remains small (≈0.05) even near critical U; SDW instability in SL subsystem preempts pairing; Hole doping suppresses inner-layer γ pocket, further weakening pairing
• Electronic Structure under Tensile Strain — Tensile strain stabilizes the γ pocket and creates favorable nesting conditions for s± pairing.
  • 材料: KTaO₃ (KTO) substrate
  • 方法: DFT and tight-binding model with adjusted lattice parameters
  • 观察: TL bonding band crosses Fermi level → small hole-like γ pocket at M point; Self-doping enhanced: SL occupancy 1.62, TL 4.38; Fermi surface nesting between electron-like σ pocket (Γ) and γ pocket (M) with wavevector near (π,π)
• Pairing Symmetry Analysis under Tension — A robust s±-wave pairing state emerges from the TL subsystem under tensile strain, driven by (π,π) nesting and strong spin fluctuations.
  • 方法: RPA on restricted TL-only model; Eigenvalue analysis of pairing vertex
  • 观察: Leading s±-wave pairing with eigenvalue λ=0.23, well separated from subleading d-wave (λ=0.17); Superconducting order parameter changes sign between σ and γ pockets; Spin susceptibility shows peak near M point → incommensurate AFM correlations; outer TL layers antiparallel, inner layer zero moment
• Doping Dependence — Superconductivity requires an optimally sized γ pocket; hole doping is detrimental, while moderate electron doping enhances pairing, but an oversized γ pocket kills superconductivity.
  • 方法: RPA with systematically varied electron/hole doping
  • 观察: Hole doping suppresses λ and enlarges γ pocket; Electron doping increases λ and shrinks γ pocket; Above ≈0.03 electron doping, γ pocket disappears and pairing collapses