Electronic structure of Ruddlesden-Popper nickelates: Strain to mimic the effects of pressure

Summary

Signatures of superconductivity under pressure have recently been reported in the bilayer La₃⁢Ni₂⁢O₇ and trilayer La₄⁢Ni₃⁢O₁₀ Ruddlesden-Popper (RP) nickelates with the general chemical formula La𝑛+1⁢Ni𝑛⁢O₃⁢𝑛+1 (𝑛 = number of perovskite layers along the 𝑐-axis). The emergence of superconductivity is always concomitant with a structural transition in which the octahedral tilts are suppressed, bringing the apical Ni-O-Ni angle to 180∘ and causing an increase in the out-of-plane 𝑑𝑧2 orbital overlap. Due to this strong interlayer coupling, a flat band of pure 𝑑𝑧2 character crosses the Fermi level. Here, using first-principles calculations, we explore biaxial strain (both compressive and tensile) as a means to mimic the electronic structure characteristics of RP nickelates (up to 𝑛=5) under hydrostatic pressure. Our findings highlight that strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure: While compressive strain brings the apical Ni-O-Ni angle closer to 180∘, it shifts the 𝑑𝑧2 flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure. In contrast, tensile strain reduces the apical Ni-O-Ni angle (to values of ∼160∘), but it recovers the flat 𝑑𝑧2 band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure. Overall, strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure in this family of materials.

Materials

• La₃Ni₂O₇
• La₄Ni₃O₁₀

Methods

• first-principles calculations

Keywords

• biaxial strain
• compressive strain
• tensile strain
• apical ni o ni angle
• dz2 flat band
• cupratelike electronic structure

Highlights

• Strain represents a promising way to tune the electronic structure of RP nickelates and could be an alternative route to achieve superconductivity at ambient pressure.

Conclusions

• Strain enables the decoupling of the structural and electronic structure effects obtained under hydrostatic pressure.
• Compressive strain brings the apical Ni-O-Ni angle closer to 180° but shifts the dz2 flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure.
• Tensile strain reduces the apical Ni-O-Ni angle but recovers the flat dz2 band at the Fermi level appearing under pressure.

Main claims

• Compressive strain suppresses octahedral tilts but shifts dz2 flat bands away from the Fermi level, giving a more cuprate-like electronic structure.
  • Evidence: Abstract: 'While compressive strain brings the apical Ni-O-Ni angle closer to 180°, it shifts the dz2 flat bands away from the Fermi energy, giving rise to a more cupratelike electronic structure.',Full text: 'Under compressive strain (−3%) the dx2−y2 bandwidth is comparable to that obtained at 30 GPa… but the bonding dz2 band around M is now shifted down in energy and fully occupied.'
• Tensile strain increases octahedral tilts but recovers the flat dz2 band at the Fermi level, mimicking the electronic structure under hydrostatic pressure.
  • Evidence: Abstract: 'tensile strain reduces the apical Ni-O-Ni angle (to values of ∼160°), but it recovers the flat dz2 band at the Fermi level appearing in the bilayer and trilayer RP nickelates under pressure.',Full text: 'under tensile strain (+3%), in spite of the very much reduced dx2−y2 bandwidth, the flat dz2 bonding band now crosses the Fermi level, giving rise to the γ pocket.'

Workflow

• structural_relaxation — Compressive strain suppresses octahedral tilts and tensile strain enhances them, mimicking pressure effects on structure.
  • Materials: La₃Ni₂O₇ bilayer (n=2); La₄Ni₃O₁₀ trilayer (n=3); La₅Ni₄O₁₃ (n=4); La₆Ni₅O₁₆ (n=5)
  • Methods: DFT structural optimization (VASP, GGA-PBE, 500 eV cutoff); Constrained in-plane lattice constants to biaxial strain; Relaxation of out-of-plane lattice constant and internal coordinates
  • Observations: Ni-O-Ni apical angle changes: compressive → closer to 180°, tensile → reduced to ≈160°; In-plane Ni-Ni distance matches 30 GPa at 3% compressive; out-of-plane matches at 3% tensile
• electronic_structure_calculation_and_analysis — Compressive strain yields cuprate-like electronic structure (no γ pocket); tensile strain recovers the γ pocket and pressure-like fermiology.
  • Materials: Relaxed structures from stage 1
  • Methods: Full-potential DFT (WIEN₂K, GGA-PBE); Maximally localized Wannier functions (WIEN₂WANNIER, WANNIER₉₀); Band structure and Fermi surface analysis
  • Observations: Under compressive strain: dx2-y2 bandwidth similar to 30 GPa, but dz2 flat band shifts below Fermi level; γ pocket absent; Under tensile strain: dz2 flat band crosses Fermi level; γ pocket present; tz⊥/tx∥ ratio increases