Layer-resolved Electronic Structure and Correlation of Low-n Square-planar Nickelates: A DFT+DMFT Prediction of Superconducting Candidates

摘要

该研究采用密度泛函理论结合动力学平均场理论（DFT+DMFT），系统分析了低 n 层四方镍酸盐的层分辨电子结构和关联效应。结果表明，未掺杂体系中 Ni-d 轨道的电子关联强度随层数增加而增强，且内层 NiO₂ 平面普遍比外层具有更强的关联性，这种层间差异源于空间电荷分布的不均匀性。针对因过度空穴掺杂而不超导的 n=2 和 n=3 化合物，提出在间隔层氧位进行 Cl 替代的电子补偿策略，通过虚拟晶体近似模拟将其名义 Ni 价调控至与最佳超导 n=6 体系相同的水平。计算显示，Cl 掺杂可显著提升低层数体系的 Ni-d 质量增强因子，使其进入强关联金属区，同时低能电子结构得以保持。该工作揭示了层分辨电子关联在超导机制中的关键作用，并预测间隔层 Cl 掺杂是将低 n 四方镍酸盐转化为潜在超导候选材料的可行路径。

材料

• La₃Ni₂O₇
• La₄Ni₃O₈
• La₅Ni₄O₁₀
• La₆Ni₅O₁₂
• La₇Ni₆O₁₄
• La₃Ni₂O₅.33Cl₀.67
• La₄Ni₃O₇.50Cl₀.50
• Nd₄Ni₃O₈
• Nd₅Ni₄O₁₀

方法

• DFT
• DMFT
• VCA
• CT-QMC

关键词

• layer resolved electronic correlations
• mass enhancement
• spacer layer cl doping
• electron compensation
• superconducting candidates
• square planar nickelates
• ni d correlations

亮点

• Proposes spacer-layer Cl doping as a continuous tuning strategy distinct from changing layer number n, leaving NiO₂ planes intact.
• Demonstrates layer-resolved correlation hierarchy with inner NiO₂ planes more correlated.
• Shows that electron compensation via Cl doping can bring low-n nickelates into the correlation regime of superconducting higher-n compounds.
• Suggests low-n compounds may be more accessible for synthesis and are promising targets for realizing superconductivity.

结论

• The undoped n=3 compound La₄Ni₃O₈ is on the over-hole-doped, weaker-correlation side, and spacer-layer Cl doping can drive low-n nickelates into the correlation regime of superconducting higher-n compounds, making them promising superconducting candidates.
• Inner NiO₂ layers are more strongly correlated than outer layers in undoped multi-layer square-planar nickelates.
• Rare-earth substitution (La vs Nd) does not substantially modify Ni-d correlations at fixed layer number.
• Spacer-layer electron compensation via Cl doping is a promising route to convert overdoped low-n square-planar nickelates into experimentally testable superconducting candidates.

主要论断

• In undoped Lan+1Ni_nO₂n+2, the Ni-d electronic correlation strength increases with layer number, and inner NiO₂ planes are always more correlated than outer ones due to inhomogeneous hole doping from spacer layers.
  • 证据: Mass enhancements: 2.87 (n=3) → 3.35 (n=6); inner vs outer in n=4: 3.01 vs 2.90 (text: Section II, Table 1, Figure 2).
• The n=3 compound (La₄Ni₃O₈) is not superconducting because its Ni-d correlations are too weak, placing it outside the strongly correlated metallic window required for superconductivity.
  • 证据: Weaker self-energy magnitude and mass enhancement (2.87) compared to superconducting higher-n compounds (text: Section II, Figures 2, 4).
• Spacer-layer Cl doping (electron compensation) can increase Ni-d correlations in low-n compounds to levels comparable with experimentally superconducting higher-n members, while preserving the low-energy electronic structure.
  • 证据: Cl-doped n=2 mass enhancement 2.98, close to undoped n=4 (2.95–3.01); Cl-doped n=3 outer layer 3.26, comparable to n=5–6 (text: Section III, Figure 3, Table 1).
• Replacing La by Nd has negligible effect on Ni-d correlations, so La-based calculations serve as valid proxies for experimentally relevant Nd-based systems.
  • 证据: La₄Ni₃O₈ vs Nd₄Ni₃O₈ inner mass enhancements: 2.87 vs 2.89; La₅Ni₄O₁₀ vs Nd₅Ni₄O₁₀: 3.01 vs 3.00 (text: Appendix B, Table 2, Figure 6).

研究流程

• sample_preparation — Crystal structures of the nickelate series were prepared as input models for DFT+DMFT calculations.
  • 材料: Undoped Lan+1Ni_nO₂n+2 (n=3-6); Nd-based analogues (Nd₄Ni₃O₈, Nd₅Ni₄O₁₀); Cl-doped low-n compounds via virtual crystal approximation (La₃Ni₂O₅.33Cl₀.67, La₄Ni₃O₇.50Cl₀.50)
  • 方法: Crystal structures taken from prior experimental measurements (implied)
  • 观察: Multi-layer square-planar structures with inequivalent Ni sites (inner and outer layers)
• measurement — DFT+DMFT calculations were performed to obtain the correlated electronic structure of the compounds.
  • 材料: Same as sample_preparation
  • 方法: Nonmagnetic density functional theory (DFT) with WIEN₂k full-potential LAPW; Charge self-consistent DFT+DMFT using eDMFT; Ni-d orbitals (dx2-y2, dz2) as correlated subspace; Continuous-time quantum Monte Carlo (CT-QMC) impurity solver in hybridization expansion; Exact double-counting correction; Maximum entropy method for real-frequency spectra
  • 观察: Self-energies, spectral functions, orbital occupancies, and mass enhancements obtained
• analysis — Layer-resolved correlations reveal that the nonsuperconducting n=3 compound is in an over-hole-doped weaker-correlation regime, while higher-n compounds enter a strongly correlated metallic window.
  • 方法: Layer- and orbital-resolved analysis of Ni-d self-energies; Estimation of quasiparticle mass enhancement from low-frequency self-energy slope; Comparison of mass enhancements across layer number and between La/Nd systems
  • 观察: Undoped series: Ni-d mass enhancement increases from 2.87 (n=3) to 3.35 (n=6); Inner NiO₂ layers systematically more correlated than outer layers (e.g., in n=4, inner 3.01 vs outer 2.90); Ni-d occupancy increases with n but does not directly control correlation strength; La and Nd compounds show nearly identical Ni-d self-energies at fixed n
• interpretation — Spacer-layer Cl doping electron-compensates the low-n compounds, driving them into the correlation regime of superconducting higher-n nickelates, making them promising superconducting candidates.
  • 方法: Simulation of spacer-layer Cl doping via virtual crystal approximation (VCA); Tuning nominal Ni valence to match the superconducting n=6 compound; Comparison of Cl-doped low-n correlations with undoped higher-n references
  • 观察: Cl-doped n=2 (La₃Ni₂O₅.33Cl₀.67): mass enhancement 2.98, comparable to undoped n=4 (La₅Ni₄O₁₀); Cl-doped n=3 (La₄Ni₃O₇.50Cl₀.50): outer-layer mass enhancement 3.26, comparable to n=5-6; inner layer 2.97; Cl doping preserves low-energy Ni-d band character and correlated metallic features