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

Summary

This study employs density functional theory combined with dynamical mean-field theory (DFT+DMFT) to systematically analyze the layer-resolved electronic structure and correlation effects in low-n tetragonal nickelates. The results reveal that the electronic correlation strength of Ni-d orbitals in undoped systems increases with layer number, and that the inner NiO₂ planes consistently exhibit stronger correlations than the outer ones, a discrepancy originating from the inhomogeneous spatial charge distribution. For the n=2 and n=3 compounds, which are non-superconducting due to excessive hole doping, an electronic compensation strategy via Cl substitution at spacer-layer oxygen sites is proposed, and virtual crystal approximation simulations tune the nominal Ni valence to match that of the optimally superconducting n=6 system. Calculations demonstrate that Cl doping significantly enhances the Ni-d mass enhancement factor in the low-layer-number systems, driving them into the strongly correlated metallic regime while preserving the low-energy electronic structure. This work highlights the critical role of layer-resolved electronic correlations in the superconductivity mechanism and predicts that spacer-layer Cl doping is a viable pathway to convert low-n tetragonal nickelates into potential superconducting candidates.

Materials

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

Methods

• DFT
• DMFT
• VCA
• CT-QMC

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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).

Workflow

• sample_preparation — Crystal structures of the nickelate series were prepared as input models for DFT+DMFT calculations.
  • Materials: 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)
  • Methods: Crystal structures taken from prior experimental measurements (implied)
  • Observations: 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.
  • Materials: Same as sample_preparation
  • Methods: 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
  • Observations: 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.
  • Methods: 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
  • Observations: 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.
  • Methods: 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
  • Observations: 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