Theoretical proposal of superconductivity in hole-doped reduced bilayer nickelate La₃Ni₂O₆: a manifestation of orbital-space bilayer model with incipient bands

摘要

本研究将多轨道Hubbard模型与双层Hubbard模型建立对应，提出轨道空间双层模型（OSBM），其中轨道能级差ΔE的作用类似于真实空间双层模型中的层间跃迁，且超导在incipient-band区域得到增强。基于此，理论预测在适当空穴掺杂下，还原型双层镍酸盐La₃Ni₂O₆可作为OSBM超导体候选。通过第一性原理构建紧束缚模型，发现由于缺乏外部顶角氧，Ni的dx²-y²轨道与其他d轨道之间存在大的ΔE。利用涨落交换近似计算表明，在incipient-band情形下，由轨道间相互作用驱动可产生s±波超导电性，超导能隙函数在dx²-y²带与其他d轨道带之间变号。同时，研究还考察了原子替代和压力下晶体结构的能量与动力学稳定性。尽管La₃Ni₂O₇与La₃Ni₂O₆化学式相似，但该工作指出后者可能实现完全不同的配对机制。

材料

• La₃Ni₂O₆
• La₃Ni₂O₇

方法

• First-principles calculations (DFT)
• FLEX approximation
• Tight-binding model
• Fluctuation exchange approximation
• Linearized Eliashberg equation
• cRPA
• Phonon calculations (Phonopy)

关键词

• orbital space bilayer model (osbm)
• incipient band
• s± wave superconductivity
• interorbital interactions
• hole doping
• t and t′ structures

亮点

• The study establishes a correspondence between the multi-orbital Hubbard model and the bilayer Hubbard model, proposing an orbital-space bilayer model (OSBM).
• Large ΔE between Ni dx2-y2 and other d orbitals is obtained due to the absence of outer apical oxygens.
• The energy difference between T and T′ structures varies under external hydrostatic pressure or internal pressure effects arising from differences in ionic radii.

结论

• s±-wave superconductivity can be enhanced by hole doping due to interorbital interactions in both T and T′ structures.
• This enhancement originates from the OSBM mechanism, where the Fermi level approaches the lower bands and the incipient-band situation is realized.
• Superconductivity is not significantly enhanced by interorbital interaction between dx2-y2 and d3z2-r2 alone; rather, it is enhanced by interorbital interactions involving all four orbitals in the lower bands.

主要论断

• Hole-doped La₃Ni₂O₆ exhibits s±-wave superconductivity driven by interorbital interactions in the incipient-band regime.
  • 证据: Abstract,Full text: FLEX calculations indicate emergence of s±-wave superconductivity driven by interorbital interactions in an incipient-band situation.
• The large orbital level offset ΔE between dx2-y2 and other d orbitals due to absence of apical oxygen is crucial for OSBM superconductivity.
  • 证据: Abstract,Full text: A large ΔE between the Ni dx2-y2 and the other d orbitals is obtained due to the absence of outer apical oxygens.
• Hole doping can induce a structural transition from T' to T structure even at ambient pressure.
  • 证据: Full text,Section III E: Enthalpy difference calculations show T structure becomes more stable upon Sr/Ba doping.

研究流程

• band_structure_calculation
  • 材料: La₃Ni₂O₆
  • 方法: DFT (PBEsol); GGA+U; QSGW
  • 观察: large orbital level offset ΔE between dx2-y2 and other d orbitals
• model_construction
  • 方法: tight-binding model from Wannier functions; cRPA for interaction parameters
• superconductivity_analysis
  • 方法: FLEX approximation; linearized Eliashberg equation
  • 观察: s±-wave superconductivity driven by interorbital interactions in incipient-band regime
• structural_stability_analysis
  • 方法: phonon dispersion calculations; enthalpy comparison
  • 观察: T and T' structures can be stabilized by doping and pressure