it Ab initio prediction of dx2-y2-wave superconductivity in infinite-layer nickelates

Summary

This study employs superconducting density functional theory to perform ab initio calculations on optimally doped infinite-layer nickelates Re₀.₈Sr₀.₂NiO₂ (Re=La, Pr, Nd), treating electron–phonon coupling, screened Coulomb repulsion, and spin fluctuations on an equal footing. The results reveal that these materials are two-band superconductors exhibiting opposite-sign d_(x²−y²)-wave pairing gaps on different Fermi surfaces; when spin fluctuations are switched off, the critical temperature drops to a negligible ~0.01 K, indicating that superconductivity is driven by spin fluctuations. On the large quasi-two-dimensional Fermi surface at the Brillouin zone center, the spin fluctuation strength is an order of magnitude larger than that of electron–phonon coupling and Coulomb repulsion, thereby dominating the pairing mechanism, whereas electron–phonon coupling plays the principal role on the small three-dimensional electron pockets at the zone corners. The nodal d-wave gap structure originates from a pronounced peak of the Lindhard response function at the zone corners. The calculated Fermi surfaces, critical temperature, nodal gaps, and quasiparticle density of states are in agreement with most experimental observations, and the predicted unconventional superconducting properties such as scanning tunneling spectra await direct experimental verification.

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

• Re₀.8Sr₀.2NiO₂ are two-band superconductors with sign reversal dx2-y2(±)-wave gap functions on different Fermi surface pockets.
  • Evidence: We find that Re₀.8Sr₀.2NiO₂ are two-band superconductors with sign reversal dx2-y2(±)-wave gap functions on the different Fermi surface (FS) pockets.
• Superconductivity is driven by spin fluctuation interaction.
  • Evidence: when the SF interaction is turned off, T_c becomes negligibly small (≈0.01 K), thus demonstrating that the superconductivity in Re₀.8Sr₀.2NiO₂ is driven by SF interaction.
• The spin fluctuation interaction is an order of magnitude stronger than electron-phonon coupling and Coulomb repulsion on the large quasi-two-dimensional Fermi surface pocket.
  • Evidence: the SF interaction is an order of magnitude stronger than both EPC and Coulomb repulsion on the large quasi-two-dimensional FS pocket around the Brillouin zone (BZ) center
• The nodal d-wave gap structure originates from pronounced peaks in the Lindhard response function at the Brillouin zone corners.
  • Evidence: The emergence of nodal d-wave gap structure is traced to the pronounced peaks in the Lindhard response function at the BZ corners.
• Calculated Fermi surfaces, critical temperature, nodal gap structure and quasiparticle density of states are consistent with most available experiments.
  • Evidence: Our calculated FS, SC critical temperature, nodal gap structure and SC quasiparticle density of states are consistent with most available experiments.
• Predicted scanning tunneling spectra for La₀.8Sr₀.2NiO₂ and Pr₀.8Sr₀.2NiO₂ are ready for experimental verification.
  • Evidence: predicted unconventional SC properties such as scanning tunneling spectra of La₀.8Sr₀.2NiO₂ and Pr₀.8Sr₀.2NiO₂ are ready for immediate experimental verifications.

Workflow

• electronic_structure_and_fermi_surface_calculation — The electronic structure of infinite-layer nickelates features a large quasi-2D Fermi surface pocket centered at Γ and small 3D pockets at the zone corners, similar to cuprates.
  • Materials: Re₀.8Sr₀.2NiO₂ (Re=La, Pr, Nd)
  • Methods: Density functional theory (DFT) with local density approximation; Optimized tetrahedron method for Brillouin zone integration; QUANTUM ESPRESSO package
  • Observations: Two bands (H and E) cross the Fermi level; Fermi surface consists of a large quasi-2D cylindrical pocket at Γ and eight small 3D electron pockets at A; Ni 3d dominates density of states near EF; FS cross-sections for LaSrNiO agree with ARPES experiments
• superconducting_properties_calculation — Re₀.8Sr₀.2NiO₂ are two-band d-wave superconductors with sign reversal dx2-y2(±) gap symmetry.
  • Materials: Re₀.8Sr₀.2NiO₂ (Re=La, Pr, Nd)
  • Methods: Superconducting density functional theory (SCDFT) gap equation solver; SCTK code; BCS-type temperature dependence fit for T_c
  • Observations: Gap function on H-band FS is d-wave with large magnitude; Gap on E-band FS pockets is d-wave with opposite sign and smaller magnitude; Calculated T_c values agree with experiments (≈18 K); Gap-to-T_c ratio 2Δ_max/kB T_c ≈ 2.3, exceeding BCS value; Quasiparticle density of states shows V-shaped gap
• pairing_mechanism_investigation — Superconductivity in infinite-layer nickelates is driven by spin fluctuations, with d-wave pairing arising from antiferromagnetic correlations due to Fermi surface nesting.
  • Materials: Re₀.8Sr₀.2NiO₂
  • Methods: Computer experiments switching off spin fluctuations and/or Coulomb repulsion; Calculation of EPC strength λ_ep, Coulomb μ*, and SF interaction V_sf on FS; Lindhard response function calculation
  • Observations: Phonon-only scenario gives T_c ≈ 1 K and s-wave gap; Adding Coulomb suppresses T_c to 0.01K; With full SF interaction, T_c ≈ 18 K and d-wave gap; V_sf is ≈10x larger than λ_ep and μ* on the H-band FS pocket; Lindhard function shows peaks at M and A points indicative of inter- and intra-pocket nesting
• experimental_comparison_and_prediction — The calculated nodal d-wave gap is consistent with most available experiments, and new STS predictions for La and Pr nickelates are ready for verification.
  • Materials: Nd₀.8Sr₀.2NiO₂ thin film STS data; Literature superfluid density and terahertz data
  • Methods: Calculation of quasiparticle density of states from SC gap function; Comparison with scanning tunneling spectroscopy (STS) spectrum
  • Observations: Calculated QPDOS of NdSrNiO matches the V-shaped STS spectrum from flat surface region; Superfluid density experiments indicate nodal gap in La and Pr nickelates, but ambiguous for Nd due to Nd magnetism; Terahertz spectroscopy on NdSrNiO suggests d-wave; Predicted STS spectra for La and Pr nickelates shown in Fig. 7(a)