Theoretical study on ambient pressure superconductivity in La₃Ni₂O₇ thin films: structural analysis, model construction, and robustness of s±-wave pairing

Summary

This study theoretically analyzes the superconductivity of La₃Ni₂O₇ thin films under ambient pressure. A model Hamiltonian is constructed based on first-principles structure optimization, with the in-plane lattice constants fixed to those of the experimental substrates (LSAT, LAO, and SLAO), and an additional model employing experimentally determined crystal structures is used. The linearized Eliashberg equation is solved using the fluctuation exchange approximation (FLEX) with full momentum- and frequency-dependent Green’s functions and pairing interactions. The results indicate that the electronic structure, including the presence or absence of the γ Fermi pocket, depends on the adopted crystal structure and whether +U corrections are included in the band calculations, yet the s±-wave pairing symmetry remains robust. This robustness primarily arises because pairing is mediated by finite-energy spin fluctuations, which are insensitive to details of the Fermi surface topology and yield a nearly momentum-independent interlayer d3z²−r² pairing gap function in the orbital representation. On the other hand, the superconducting transition temperature of the thin films (approximately 40 K) is about half that of bulk material under pressure (approximately 80 K). Within the FLEX framework, this can only be understood by adopting a model with a small interlayer hopping parameter |t⊥| derived from experimentally determined crystal structures, although other contributing factors cannot be ruled out.

Materials

• La₃Ni₂O₇

Methods

• DFT
• FLEX
• Wannier function
• linearized Eliashberg equation
• PBEsol+U

Keywords

• s± wave pairing
• spin fluctuations
• gamma pocket
• bilayer hubbard model
• tc reduction

Highlights

• Ambient pressure superconductivity in La₃Ni₂O₇ thin films is achieved
• The interlayer Ni-O-Ni bond angle deviation from 180° has small impact on superconductivity within FLEX
• The gap function is nearly momentum-independent for interlayer d3z2-r2 pairing
• FLEX results agree with cDMFT, DCA, and VMC non-perturbative approaches
• The origin of the large discrepancy between theoretical and experimental crystal structures remains an open question

Conclusions

• s±-wave pairing symmetry remains robust regardless of details in band structure
• The robustness is due to finite energy spin fluctuations that are insensitive to Fermi surface topology
• The reduction of T_c in thin films to about 40 K from about 80 K in bulk can be explained by the small interlayer hopping derived from the experimentally determined crystal structure

Main claims

• s±-wave pairing symmetry is robust in La₃Ni₂O₇ thin films under ambient pressure, regardless of the presence or absence of the γ-pocket and details of the crystal structure.
  • Evidence: FLEX calculations for three different crystal structures (theoretical I₄/mmm, theoretical I₄/m, experimental I₄/m) all yield s±-wave pairing.,Gap function in orbital representation is nearly momentum-independent for interlayer d3z2-r2 pairing.,Finite-energy spin fluctuations, insensitive to Fermi surface topology, mediate the pairing (see Fig. 9 and discussion in Sec. III.3.3).
• The reduced T_c (≈40 K) in thin films compared to pressurized bulk (≈80 K) can be understood within FLEX only if the experimentally determined crystal structure (small |t⊥| ≈ 0.4 eV) is adopted.
  • Evidence: Eigenvalue of linearized Eliashberg equation (λ) is significantly smaller for experimental structure than for theoretical structures (Fig. 8(a)).,λ decreases monotonically with reducing interlayer hopping |t⊥| (Fig. 8(c)).,Theoretical structures give λ comparable to pressurized bulk, failing to explain reduced T_c.
• The pairing mechanism is dominated by finite-energy (>0.6 eV) spin fluctuations, not by Fermi surface nesting.
  • Evidence: Dynamical spin susceptibility shows that low-energy spin fluctuations are momentum-dependent and enhanced by better nesting, but high-energy fluctuations are nearly momentum-independent (Fig. 9).,λ is not enhanced by the appearance of the γ-pocket (better nesting) but decreases when |t⊥| is small (where nesting is worse).,Gap function in orbital representation is nearly momentum-independent, consistent with pairing mediated by finite-energy fluctuations.

Workflow

• First-principles structural optimization — Structural parameters obtained as function of in-plane lattice constant; bond angle approaches 180° for smaller lattice constant
  • Materials: La₃Ni₂O₇ thin films; substrates (LSAT, LAO, SLAO)
  • Methods: DFT structural optimization with PBEsol functional; projector augmented wave (PAW) method; VASP code
  • Observations: Determined lattice constants and Ni-O-Ni bond angles as function of substrate
• Tight-binding model construction — Interlayer hopping integral |t⊥| differs significantly between theoretically determined and experimentally determined structures
  • Materials: DFT-optimized structures; experimentally determined structure (Ref. [24])
  • Methods: Extraction of Wannier functions (WANNIER₉₀); Two-orbital bilayer Hubbard model with Ni-3dx2-y2 and 3d3z2-r2 orbitals
  • Observations: Hopping parameters and level offset extracted; interlayer hopping |t⊥| ≈ 0.6 eV for theoretical structures, ≈0.4 eV for experimental structure
• Fluctuation exchange approximation (FLEX) calculation — s±-wave pairing symmetry robust across different crystal structures and Fermi surface details
  • Materials: Two-orbital bilayer Hubbard models for three crystal structures (theoretical I₄/mmm, theoretical I₄/m, experimental I₄/m)
  • Methods: FLEX approximation; self-consistent calculation of self-energy and spin-fluctuation-mediated pairing interaction; full momentum and frequency dependence of Green's function
  • Observations: Eigenvalue of linearized Eliashberg equation obtained; gap function in orbital and band representation
• Analysis of superconductivity and pairing mechanism — Finite-energy spin fluctuations mediate robust s±-wave pairing; absence of γ-pocket does not suppress superconductivity
  • Materials: FLEX results for all models
  • Methods: Solution of linearized Eliashberg equation; analytical continuation of dynamical spin susceptibility via Padé approximation; analysis of gap function and pairing glue
  • Observations: Gap function in orbital representation nearly momentum-independent for interlayer pairing; spin susceptibility shows finite-energy spin fluctuations as pairing glue