Nearly perfect Fermi surface nesting in hole-doped La₃Ni₂O₇ enables bulk superconductivity without pressure or strain

Summary

B_y combining density functional theory, dynamical mean-field theory, and the random phase approximation to solve the superconducting gap equation, researchers have discovered that hole-doped layered nickel oxide La_3-xSr_xNi₂O₇ can achieve bulk superconductivity under ambient pressure. When the doping concentration x approaches 0.4, the γ Fermi pocket derived from the Ni-d₃z²_-r² orbital evolves from a circular to a diamond shape and expands to half the Brillouin zone, forming a nearly perfect Fermi surface nesting with an optimal nesting vector Q=(π, π). This structure significantly enhances antiferromagnetic spin fluctuations, elevating the superconducting eigenvalue to experimentally observable levels without the need for high pressure or strain. This work elucidates the mechanism by which hole doping modulates the shape and size of the Fermi pocket, providing a theoretical foundation and an experimentally feasible pathway for realizing the long-sought bulk superconductivity in such materials under ambient conditions.

Materials

• La_3-xSr_xNi₂O₇

Methods

• DFT
• DMFT
• Random phase approximation (RPA)
• Linearized gap equation

Keywords

• fermi surface nesting
• antiferromagnetic spin fluctuations
• superconducting eigenvalue
• hole doping
• γ pocket
• s± wave pairing
• ambient pressure superconductivity

Highlights

• Provides a robust mechanism and an experimentally feasible route to inducing the long-sought bulk superconductivity in La₃Ni₂O₇ without pressure or strain.
• The diamond-shaped γ pocket at x=0.4 exhibits nearly perfect Fermi surface nesting, substantially increasing the leading superconducting eigenvalue to an experimentally observable level.
• First proposal to achieve ambient-pressure bulk superconductivity in RP nickelates via hole doping (Sr substitution).
• Identifies optimal doping x=0.4 where the γ pocket exhibits nearly perfect nesting, leading to a substantial increase in superconducting eigenvalue.

Conclusions

• A substantially increased superconducting eigenvalue is found in bulk La_3-xSr_xNi₂O₇ at x=0.4 under ambient pressure, yielding observable superconductivity.
• The underlying mechanism is that hole doping induces a Ni-d3z2-r2-derived γ pocket that evolves from circular to diamond-shaped and expands to span half of the Brillouin zone, resulting in nearly perfect Fermi surface nesting with optimal nesting vector Q=(π,π), which enhances antiferromagnetic spin fluctuations and induces unconventional superconductivity.
• Hole doping induces a Ni-dz2-derived γ pocket on the Fermi surface; at x=0.4, the pocket becomes diamond-shaped and spans half of the Brillouin zone, achieving nearly perfect Fermi surface nesting with vector Q=(π,π).
• This nesting strongly enhances antiferromagnetic spin fluctuations and substantially increases the leading superconducting eigenvalue to an experimentally observable level.
• Provides a mechanism and feasible experimental route to achieving bulk superconductivity in La₃Ni₂O₇ without pressure or strain.

Main claims

• Hole doping induces a Ni-d3z2-r2 derived γ pocket that evolves from circular to diamond-shaped as x approaches 0.4, resulting in nearly perfect Fermi surface nesting.
  • Evidence: Full text: 'As x approaches 0.4, the γ pocket evolves from circular to diamond-shaped and expands to span half of the Brillouin zone, resulting in nearly perfect Fermi surface nesting with the optimal nesting vector Q = (π, π).'
• This nesting enhances antiferromagnetic spin fluctuations and substantially increases the leading superconducting eigenvalue to yield experimentally observable superconductivity at ambient pressure.
  • Evidence: Abstract: '…strongly enhances antiferromagnetic spin fluctuations and substantially increases the leading superconducting eigenvalue to a level at which superconductivity becomes experimentally observable.'
• Hole doping at x=0.4 induces nearly perfect Fermi surface nesting and enhances antiferromagnetic spin fluctuations, leading to observable superconductivity
  • Evidence: From abstract: 'As x approaches 0.4, the γ pocket evolves from circular to diamond-shaped and expands to span half of the Brillouin zone, resulting in nearly perfect Fermi surface nesting with the optimal nesting vector Q = (π, π). This, in turn, strongly enhances antiferromagnetic spin fluctuations and substantially increases the leading superconducting eigenvalue'

Workflow

• Electronic structure calculation — Hole doping serves as a tuning parameter for size and shape of the γ pocket
  • Materials: La₃Ni₂O₇ crystal structure
  • Methods: Density functional theory (DFT); Dynamical mean-field theory (DMFT)
  • Observations: Hole doping induces a Ni-d3z2-r2 derived γ pocket on Fermi surface
• Superconducting gap equation solution — Leading superconducting eigenvalue increases to experimentally observable level
  • Materials: Bilayer two-orbital model fitted to DFT bands
  • Methods: Random phase approximation (RPA); Linearized gap equation
  • Observations: Nearly perfect Fermi surface nesting with nesting vector Q=(π,π); Enhanced antiferromagnetic spin fluctuations
• density_functional_theory_calculations — Obtained non-interacting electronic structure
  • Materials: La_3-xSr_xNi₂O₇ bulk
  • Methods: DFT with PBE functional
  • Observations: band structure; Fermi surface
• dynamical_mean_field_theory_calculations — Obtained interacting electronic structure
  • Materials: Bilayer two-orbital model
  • Methods: DMFT with Slater-Kanamori interaction
  • Observations: quasiparticle dispersion; quasiparticle weight
• random_phase_approximation_analysis — At x=0.4, nearly perfect Fermi surface nesting enhances spin fluctuations and superconducting eigenvalue
  • Materials: DMFT quasiparticle energies
  • Methods: RPA+DMFT for spin susceptibility and pairing potential
  • Observations: leading superconducting eigenvalue; spin susceptibility