Pairing mechanism and superconductivity in pressurized La₅Ni₃O₁₁

Summary

The discovery of superconductivity (SC) with critical temperature T_c above the boiling point of liquid nitrogen in pressurized La₃Ni₂O₇ has sparked a surge of exploration of high-T_c superconductors in the Ruddlesden-Popper (RP) phase nickelates. More recently, the RP phase nickelate La₅Ni₃O₁₁, which hosts a layered structure with alternating bilayer and single-layer NiO₂ planes, has been reported to accommodate SC under pressure, exhibiting a dome-shaped pressure dependence with the highest T_c ≈ 64 K, capturing a lot of interest. Here, using density functional theory (DFT) and random phase approximation (RPA) calculations, we systematically study the electronic properties and superconducting mechanism of this material. Our DFT calculations yield a band structure including two nearly decoupled sets of sub-band structures, with one set originating from the bilayer subsystem and the other from the single-layer one. RPA-based analysis demonstrates that SC in this material occurs primarily within the bilayer subsystem exhibiting an s± wave pairing symmetry similar to that observed in pressurized La₃Ni₂O₇, while the single-layer subsystem mainly serves as a bridge facilitating the inter-bilayer phase coherence through the interlayer Josephson coupling (IJC). Since the IJC thus attained is extremely weak, it experiences a prominent enhancement under pressure, leading to the increase of the bulk T_c with pressure initially. When the pressure is high enough, the T_c gradually decreases due to the reduced density of states on the γ-pocket. In this way, the dome-shaped pressure dependence of T_c observed experimentally is naturally understood.

Materials

• La₅Ni₃O₁₁

Methods

• DFT
• random phase approximation (RPA)

Keywords

• dome shaped pressure dependence
• s± wave pairing symmetry
• interlayer josephson coupling (ijc)
• γ pocket density of states

Highlights

• The single-layer subsystem serves as a bridge for inter-bilayer phase coherence.
• Pressure initially enhances IJC then reduces DOS on γ-pocket.

Conclusions

• Superconductivity in pressurized La₅Ni₃O₁₁ occurs primarily in the bilayer subsystem with s± wave pairing symmetry, and the single-layer subsystem facilitates inter-bilayer coherence via IJC, leading to a dome-shaped T_c vs pressure.

Main claims

• Superconductivity in La₅Ni₃O₁₁ primarily originates from the bilayer subsystem with s±-wave pairing symmetry.
  • Evidence: Abstract: 'RPA-based analysis demonstrates that SC in this material occurs primarily within the bilayer subsystem exhibiting an s± wave pairing symmetry',Full text: RPA results show largest pairing eigenvalue for s-wave (Fig. 4(c)), gap function with s± pattern (Fig. 4(d)).
• The single-layer subsystem serves mainly as a bridge for inter-bilayer Josephson coupling, not as the primary superconducting layer.
  • Evidence: Abstract: 'the single-layer subsystem mainly serves as a bridge facilitating the inter-bilayer phase coherence through the interlayer Josephson coupling',Full text: DMFT studies show SL subsystem is nearly Mott-insulating and does not carry superconductivity.
• The dome-shaped pressure dependence of T_c results from the competition between decreasing intralayer pairing (due to reduced DOS) and increasing interlayer Josephson coupling (due to pressure-enhanced hopping).
  • Evidence: Abstract: 'the dome-shaped pressure dependence of T_c observed experimentally is naturally understood',Full text: equations (13) and (14) modeling T_c(P) and Fig. 6(d) reproducing dome-shaped behavior.

Workflow

• DFT Band Structure Calculations — The electronic structure comprises nearly independent BL and SL subsystems.
  • Materials: La₅Ni₃O₁₁ (SL-BL hybrid RP nickelate); P₄/mmm phase at 12 GPa
  • Methods: First-principles DFT (VASP); GGA-PBE exchange-correlation; Hubbard U=3.5 eV for Ni 3d
  • Observations: Band structure with two nearly decoupled sub-band sets: bilayer (BL) and single-layer (SL) subsystems; Five Fermi surface pockets: α, α', β, γ, γ'
• Tight-Binding Model Construction — The two subsystems can be treated separately due to very weak inter-subsystem coupling.
  • Materials: Wannier downfolding from DFT; Ni-dz2 and dx2-y2 orbitals (6 orbitals total)
  • Methods: Maximally localized Wannier functions (WANNIER₉₀)
  • Observations: Hopping between SL and BL is extremely weak (max ≈0.01 eV); Decoupling allows separate treatment of subsystems
• RPA Analysis of Superconducting Pairing — Superconducting pairing in La₅Ni₃O₁₁ primarily originates from the bilayer subsystem with s±-wave symmetry, similar to pressurized La₃Ni₂O₇.
  • Materials: BL subsystem (4 orbitals)
  • Methods: Multi-orbital random phase approximation (RPA); Linearized gap equation for pairing eigenvalues
  • Observations: Spin susceptibility peaks at Q₁=(π,0) and Q₂; Leading pairing symmetry is s-wave (s± pattern); Pairing mainly on α and γ pockets with opposite signs
• Analysis of Pressure Dependence — The dome-shaped pressure dependence of T_c in La₅Ni₃O₁₁ arises from the competition between decreasing intralayer pairing and increasing interlayer Josephson coupling.
  • Materials: TB parameters for 12-42 GPa
  • Methods: DFT for pressure-dependent hopping; Model for interlayer Josephson coupling (IJC); Relation: T_c ∝ ω_D e-1/λ/sqrt(ln(32/η))
  • Observations: Pairing eigenvalue λ decreases with pressure (due to reduced DOS on γ pocket); IJC (η) increases exponentially with pressure; Combined effect gives dome-shaped T_c(P)