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

Summary

B_y combining density functional theory and random phase approximation, the effects of compressive and tensile strain on the superconductivity of 1313-La₃Ni₂O₇ thin films are systematically investigated. A self-doping effect is found between the monolayer and trilayer blocks regardless of compressive or tensile strain, and it is most pronounced under tensile strain. Under compressive strain imposed by an LSAO substrate, even considering hole doping from strontium ion migration in the substrate, superconductivity is difficult to appear, consistent with experiments. However, under tensile strain from a KTO substrate, a band in the trilayer subsystem that originally did not cross the Fermi level shifts downward, giving rise to a small hole-type γ pocket at the M point, which is connected to a small electron-type σ pocket at the Γ point by a near-(π,π) wavevector. Random phase approximation calculations reveal that the trilayer subsystem can then form a stable s±-wave pairing state, with the order parameter reversing sign between these two pockets. Further analysis indicates that the size of the γ pocket is crucial for pairing, and an excessively large γ pocket suppresses superconductivity. This work predicts a strain-driven electronic structure reconstruction and proposes a design principle to realize superconductivity in 1313-La₃Ni₂O₇ under ambient pressure through tensile strain engineering.

Materials

• 1313-La₃Ni₂O₇

Methods

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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• Under compressive strain (LSAO substrate), superconductivity is unlikely in 1313-LNO films, even with hole doping from Sr migration.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.

Workflow

• Model Construction — Under compressive strain, the electronic structure lacks the γ pocket, and charge self-doping redistributes electrons between SL and TL blocks.
  • Materials: 1313-La₃Ni₂O₇ thin film; LSAO substrate; DFT code (VASP); WANNIER₉₀
  • Methods: Density functional theory (DFT); Maximally localized Wannier functions; Tight-binding model (8-band Ni-eg orbitals)
  • Observations: 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.
  • Methods: Multi-orbital random-phase approximation (RPA); Restricted TL-only model (SL Coulomb elements set to zero); Spin susceptibility and pairing eigenvalue equation
  • Observations: 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.
  • Materials: KTaO₃ (KTO) substrate
  • Methods: DFT and tight-binding model with adjusted lattice parameters
  • Observations: 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.
  • Methods: RPA on restricted TL-only model; Eigenvalue analysis of pairing vertex
  • Observations: 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.
  • Methods: RPA with systematically varied electron/hole doping
  • Observations: 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