Microscopic Origin of Pressure-Enhanced and Robust Superconductivity in Infinite-Layer La₀.8Sr₀.2NiO₂

Summary

Combining first-principles calculations, a pressure-dependent two-orbital model, self-consistent FLEX calculations, and the linearized Eliashberg equation, this study reveals the microscopic mechanism of pressure-enhanced and robust superconductivity in infinite-layer La₀.₈Sr₀.₂NiO₂. The results show that increasing pressure enlarges the kinetic energy scale, reduces the effective correlation strength U/t, enhances interlayer hybridization, and transfers holes from the La/Sr charge reservoir to the correlated Ni region; at low pressure, the enlarged kinetic energy scale and the approach to optimal intermediate coupling promote pairing, whereas at high pressure, pressure-induced self-doping drives the system into the overdoped regime and suppresses superconductivity, thereby forming a broad superconducting dome. Although compression significantly three-dimensionalizes the Fermi surface, the spin susceptibility relevant to pairing depends weakly on q_z and still peaks mainly near (π,π), so the Ni dx²−y²-dominated d-wave pairing state remains stable over the calculated pressure range. This constrained low-energy pairing framework explains the unusual robustness of superconductivity in this material under megabar-level compression.

Materials

• La₀.8Sr₀.2NiO₂

Methods

• DFT
• maximally localized Wannier functions (MLWFs)
• constrained random-phase approximation (cRPA)
• fluctuation-exchange approximation (FLEX)
• linearized Eliashberg equation
• pressure-dependent two-orbital model

Keywords

• superconducting dome
• pressure enhanced superconductivity
• kinetic energy scale
• effective correlation strength u/t
• interlayer hybridization
• hole self doping
• overdoped regime
• spin susceptibility
• d wave pairing
• ni dx2 y2 orbital
• three dimensional fermi surface

Highlights

• A unified microscopic interpretation of the broad superconducting dome is provided through the competition among kinetic-energy enhancement, reduced correlation strength, and hole self-doping.
• Interlayer hybridization strongly reshapes the Fermi surface but has only a secondary effect on pairing in this minimal low-energy framework.
• The restricted low-energy manifold of infinite-layer nickelates is proposed as a minimal platform for testing theories of unconventional superconductivity.
• Reducing the nominal Sr concentration may avoid the overdoped regime under pressure and potentially increase the maximum high-pressure T_c.

Conclusions

• Pressure increases the kinetic-energy scale, reduces the effective correlation strength U/t, strengthens interlayer hybridization, and transfers holes from the La/Sr-derived charge reservoir to the correlated Ni sector.
• The initial enhancement of pairing originates from the increasing kinetic-energy scale and the approach to optimal intermediate coupling.
• The high-pressure suppression of superconductivity is primarily caused by pressure-induced hole self-doping that drives the Ni band into the overdoped regime.
• Despite pronounced three-dimensionalization of the Fermi surface, the pairing-relevant spin susceptibility remains weakly dependent on qz and peaked near (π,π).
• The Ni-dx2-y2-dominated d-wave pairing state remains stable over the calculated pressure range, explaining the robustness of superconductivity under megabar compression.

Main claims

• Pressure increases the kinetic-energy scale, reduces the effective correlation strength U/t, strengthens interlayer hybridization, and transfers holes from the La/Sr charge reservoir to the correlated Ni sector.
  • Evidence: Pressure increases the kinetic-energy scale, reduces Ux/t1, strengthens interlayer hybridization, and transfers holes from the La/Sr-derived charge reservoir to the correlated Ni sector.,The nearest-neighbor in-plane hopping increases from its ambient-pressure value to its 210 GPa value; interlayer hopping increases even more strongly; the screened interaction remains nearly constant; hole self-doping increases.
• The pressure dependence of superconductivity is dome-shaped: increasing kinetic scale and approach to optimal intermediate coupling enhance pairing at low pressure, while pressure-induced self-doping into the overdoped regime suppresses pairing at high pressure.
  • Evidence: At low pressure, the increasing kinetic scale and the approach to optimal intermediate coupling enhance pairing, while self-doping remains weak. Above approximately GPa, accumulated hole self-doping drives the Ni band further into the overdoped regime and generates the descending side of the dome.,The leading Eliashberg eigenvalue initially increases rapidly with pressure, reaches a maximum around GPa, and then decreases, reproducing the qualitative dome-shaped pressure dependence observed experimentally.
• The Ni-dx2-y2-dominated d-wave pairing state remains stable over the calculated pressure range despite pronounced three-dimensionalization of the Fermi surface, because pairing-relevant spin fluctuations remain weakly dependent on q_z and peaked near (pi,pi).
  • Evidence: The leading eigenfunction has d-wave symmetry at every pressure investigated.,The spin susceptibility remains peaked near the in-plane wave vector at both pressures and on both q_z planes, and its weak q_z dependence shows that the pairing-relevant spin fluctuations remain predominantly in-plane.,The active correlated sector of La₀.8Sr₀.2NiO₂ remains dominated by a single Ni-dx2-y2 orbital, while the dz2 orbital primarily mediates interlayer dispersion.

Workflow

• model_construction — The low-energy electronic structure is captured by a two-band tight-binding model dominated by the Ni-dx2-y2 orbital, with La/Sr reservoir effects incorporated as a pressure-dependent carrier concentration.
  • Materials: La₀.8Sr₀.2NiO₂; two-orbital Ni-dx2-y2/Ni-dz2 tight-binding Hamiltonian; La/Sr-derived charge reservoir
  • Methods: DFT structure optimizations; virtual-crystal approximation for Sr substitution; maximally localized Wannier functions; constrained random-phase approximation
  • Observations: Wannier Hamiltonian reproduces Ni-dominated DFT bands near Fermi level; Ni-dx2-y2 forms the principal correlated Fermi surface; Ni-dz2 orbital remains nearly fully occupied
• parameter_evolution_extraction — Pressure increases the kinetic-energy scale, reduces U/t, enhances interlayer hybridization, and transfers holes from the La/Sr charge reservoir to the Ni sector.
  • Materials: pressure-dependent two-orbital model; cRPA screened interaction
  • Methods: extracting pressure-dependent hopping parameters from the MLWF Hamiltonian; evaluating screened interaction with cRPA; determining effective hole concentration from La/Sr electron pocket expansion
  • Observations: in-plane hopping increases with pressure; interlayer hopping increases more strongly; screened interaction remains nearly constant; hole self-doping increases
• pairing_calculation — The calculated pairing tendency reproduces the experimentally observed dome-shaped pressure dependence.
  • Materials: FLEX self-energy and susceptibilities; linearized Eliashberg equation
  • Methods: self-consistent FLEX calculations; linearized Eliashberg equation for spin-singlet pairing; selective variation of model parameters; temperature-dependent eigenvalue calculations
  • Observations: leading eigenvalue increases initially, peaks around optimal pressure, then decreases; eigenfunction has d-wave symmetry at every pressure investigated; fixing ambient-pressure carrier concentration removes most high-pressure suppression; fixing interlayer hopping parameters only modestly changes the eigenvalue
• interpretation — The superconducting dome and robustness under megabar compression arise from a preserved low-energy Ni-dx2-y2-dominated d-wave pairing framework.
  • Materials: Ni-dx2-y2 spectral function; static spin susceptibility
  • Methods: comparison of spectral function and spin susceptibility at ambient and high pressure; identification of dominant pairing trends from selective parameter variation
  • Observations: spin susceptibility remains peaked near (pi,pi) and weakly q_z dependent; pressure substantially modifies the one-particle electronic structure; correlated sector remains dominated by a single Ni-dx2-y2 orbital