Pairing symmetry and superconductivity in La₃Ni₂O₇ thin films

Summary

Using renormalized mean-field theory based on a bilayer (t-J) model incorporating (dz2) and (dx2-y2) orbitals, this study systematically investigates the superconducting pairing symmetry of La₃Ni₂O₇ thin films. Self-consistent solutions reveal (s_±)-wave pairing driven by strong interlayer superexchange coupling of the (dz2) orbital, consistent with the pressurized bulk case, and successfully reproduce the nodeless superconducting gap structure on the (beta) Fermi surface pocket observed by angle-resolved photoemission spectroscopy, yielding a calculated superconducting transition temperature of approximately 60 K in agreement with experiments. Orbital-resolved analysis demonstrates that the nodeless character of the (beta) pocket arises from the cooperative interlayer pairing of (dz2) and (dx2-y2) orbitals, while in-plane pairing between these orbitals generates a (d)-wave component that further enhances the dominant (s_±)-wave order. The work elucidates the diverse cooperative and competitive relationships among different pairing channels on the complex Fermi surfaces of La₃Ni₂O₇ films and discusses the potential modulation of pairing symmetry by factors such as substrate strain and oxygen vacancies, providing a crucial theoretical basis for understanding the superconducting mechanism in nickelates.

Materials

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Methods

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Keywords

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Highlights

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Conclusions

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Main claims

• Superconductivity in La₃Ni₂O₇ thin films has s±-wave pairing symmetry.
  • Evidence: RMFT self-consistent solution yields s±-wave form factors,Figure 2(a) shows nodeless β pocket and opposite-sign pockets
• The pairing is driven by strong interlayer superexchange coupling of the dz2 orbital.
  • Evidence: Interlayer pairing bond Δ_perp^z-z is dominant,Large gap on α pocket reflects strong J_perp
• The nodeless gap on the β pocket results from interlayer pairing within both dz2 and dx2-y2 orbitals.
  • Evidence: Orbital-resolved gaps show joint contributions with no node on β pocket (Fig. 3(b,d)),Both orbitals exhibit same sign phase on that pocket
• An inplane inter-orbital d-wave pairing component further enhances the dominant s±-wave.
  • Evidence: Δx2-y2 inplane channel develops d-wave form factor,Projection onto Fermi surface shows gap amplification (Appendix A)
• The calculated T_c of ≈60 K is in agreement with experimental values for thin films.
  • Evidence: Gap magnitudes drop to zero at T_c≈60 K in RMFT,Experimental reports give 40–60 K T_c under ambient pressure

Workflow

• Model construction — The model captures essential electronic structure for studying superconductivity in La₃Ni₂O₇ thin films.
  • Materials: bilayer two-orbital t-J model; DFT-derived tight-binding Hamiltonian; Heisenberg exchange parameters (J_perp, J_parallel)
  • Methods: exact diagonalization for superexchange coupling estimation; Gutzwiller renormalized mean-field theory framework
  • Observations: effective low-energy Hamiltonian suitable for RMFT
• RMFT self-consistent calculation — Self-consistent RMFT yields an s±-wave superconducting ground state.
  • Materials: mean-field order parameters; pairing bond amplitudes
  • Methods: self-consistent solution of RMFT equations; application of Gutzwiller renormalization factors
  • Observations: s±-wave pairing symmetry; dominant interlayer s± driven by dz2 orbital; finite pairing bond magnitudes
• Gap structure analysis — The nodeless β pocket gap arises from cooperative interlayer pairing of both orbitals, and an additional d-wave component enhances the order.
  • Materials: superconducting gap matrices projected on Fermi surface; orbital-resolved gap components
  • Methods: gap projection onto Fermi pockets (α, β, γ); orbital decomposition of pairing bonds
  • Observations: nodeless gap on β pocket, sign flip on γ pocket; nodeless character originates from interlayer dz2 and dx2-y2 pairs; inplane inter-orbital d-wave component enhances s±
• Interpretation and experimental comparison — The calculated T_c and gap structure support the s±-wave scenario and the role of interlayer superexchange, consistent with thin-film experiments.
  • Materials: temperature dependence of gap values; experimental ARPES gap and T_c data
  • Methods: RMFT at varying temperatures; comparison with experimental T_c and gap anisotropy
  • Observations: gaps vanish sharply at T_c ≈ 60 K, comparable to experimental 40–60K; gap anisotropy on β pocket consistent with ARPES nodeless shape