Electron Doping of La₃Ni₂O₇ Thin Films: Candidate Metal Dopants and Their Potential Impact on Superconductivity

摘要

通过第一性原理密度泛函理论计算，系统研究了双层Ruddlesden-Popper型镍酸盐La₃Ni₂O₇薄膜中四价元素替代的电子掺杂效应。研究发现，与铜氧化物不同，铈（Ce）掺杂难以有效向低能带引入电子载流子，而锆（Zr）、铪（Hf）和钍（Th）可作为高效电子掺杂剂。这些元素替代能显著增大Ni-dz²轨道间的层间跳跃积分t⊥，可能增强层间超交换耦合J⊥，进而潜在提高超导转变温度T_c。利用约束随机相位近似评估了相互作用参数，发现电子掺杂使得低能轨道（包括Ni-dx²-y²和dz²及其杂化氧轨道）的占据增加，并改变了层内与层间轨道的电子填充比例。结构分析显示，掺杂剂离子半径的差异导致Ni-O键长变化，其中Zr和Hf引起晶格收缩，而Th掺杂效果最强。结果表明，Zr、Hf、Th是实现La₃Ni₂O₇电子掺杂的候选材料，为澄清该体系中电子配对机制的争议提供了新途径。

材料

• La₃Ni₂O₇
• Ce-doped La₃Ni₂O₇
• Zr-doped La₃Ni₂O₇
• Hf-doped La₃Ni₂O₇
• Th-doped La₃Ni₂O₇

方法

• DFT
• Constrained random-phase approximation (cRPA)

关键词

• electron doping
• interlayer hopping t⊥
• superexchange coupling j⊥
• pairing mechanism
• hund's coupling

亮点

• Unlike in cuprates, Ce doping fails to introduce electron carriers into low-energy bands in La₃Ni₂O₇.
• Electron doping alters the electron filling ratio between in-plane and interlayer orbitals, which may affect pairing symmetry.
• First systematic DFT study of electron doping in La₃Ni₂O₇ thin films, identifying promising dopants.

结论

• Cerium (Ce) doping is inefficient for introducing electron carriers into low-energy bands of La₃Ni₂O₇.
• Zirconium (Zr), hafnium (Hf), and thorium (Th) act as effective electron dopants in La₃Ni₂O₇ thin films.
• Electron doping significantly increases the interlayer hopping t⊥ between dz2 orbitals, potentially enhancing superexchange coupling J⊥ and possibly T_c.
• These candidate dopants provide a route to clarify the debate on pairing mechanisms in bilayer nickelates.

主要论断

• Cerium doping does not effectively introduce electron carriers into La₃Ni₂O₇, unlike in cuprates.
  • 证据: low-energy bands of La₃Ni₂O₇:Ce almost coincide with pristine La₃Ni₂O₇,Ni-O bond lengths remain unchanged upon Ce doping,Ce retains largelyCe₃+ valence state, not Ce₄+
• Zirconium, hafnium, and thorium are effective electron dopants for La₃Ni₂O₇, enhancing interlayer hopping and potentially increasing T_c.
  • 证据: Ni-dz2 and dx2-y2 bands shift downward in energy upon doping,interlayer hopping integral t⊥ increases significantly (ratio t⊥/t‖ from 1.7 to 2.5),Bader charge analysis shows increased charge supply to the system,Hund's coupling increases relative to Hubbard U, potentially boosting superconductivity

研究流程

• model_setup — Thin film models with various dopants and substrates are constructed for DFT calculations.
  • 材料: one-unit-cell thick La₃Ni₂O₇ film on substrates (LaAlO₃, SrTiO₃, NdGaO₃)
  • 方法: density functional theory (DFT) with VASP; projector augmented wave (PAW) pseudopotentials; PBE exchange-correlation functional; DFT+U with U=3.5 eV for Ni-3d orbitals
  • 观察: structures relaxed to convergence
• electronic_structure_calculation — Zr, Hf, Th are effective electron dopants; Ce is not.
  • 材料: La₃Ni₂O₇ thin films with dopants
  • 方法: DFT band structure calculations; density of states (DOS) analysis
  • 观察: Ce doping shows no significant change in low-energy bands; Zr, Hf, Th doping shifts Ni-dz2 and dx2-y2 bands downward in energy; orbital occupancy increases for both in-plane and out-of-plane orbitals
• structural_analysis — Structural changes correlate with dopant ionic radii and affect electron doping efficiency.
  • 材料: relaxed atomic structures
  • 方法: analysis of Ni-O bond lengths and La-O bonds
  • 观察: Ce doping leaves Ni-O bonds nearly unchanged; Zr/Hf doping causes lattice contraction due to smaller ionic radii; Th doping also contracts lattice but has stronger electron doping effect
• charge_and_orbital_analysis — Electron doping modifies charge distribution and orbital occupancy, with Th showing strongest effect.
  • 材料: DFT charge densities
  • 方法: Bader charge analysis; integrated DOS for orbital occupancy
  • 观察: Th supplies additional charge to the system; Sr removes charge; electron doping increases occupancy of low-energy orbitals; relative ratio of carriers in in-plane vs out-of-plane orbitals varies with dopant
• tight_binding_and_interaction_parameters — Electron doping enhances interlayer hopping and Hund's coupling, potentially raising T_c.
  • 材料: DFT band structures
  • 方法: Wannier90 tight-binding fitting; constrained random-phase approximation (cRPA) for Hubbard U and Hund's coupling
  • 观察: interlayer hopping t⊥ increases from ≈1.7 to ≈2.5 relative to in-plane hopping; Hund's coupling increases relative to average Hubbard U