From perovskite to infinite-layer nickelates: hole concentration from x-ray absorption

摘要

该研究利用软X射线吸收光谱系统研究了PrNiO_x薄膜在拓扑还原各中间阶段（x=2-3）的变化。通过比较Ni-L边实验光谱与单团簇及双团簇配位场模型计算，发现所有样品均未呈现纯d⁹电子构型。基于电荷和规则的定量分析表明，即便在最大程度还原的薄膜中，镍3d空穴的平均数仍为1.35，而超导样品的空穴数更高，这质疑了此前关于空穴掺杂极限的假设。还原过程中氧K边吸收光谱的同步变化表明，即使在最还原的薄膜中也存在氧2p空穴。综合结果指出，自掺杂效应与氧非化学计量共同导致了复杂的空穴掺杂机制。

材料

• PrNiOx
• NdNiO₂
• NiO
• Nd_1-ySr_yNiO₂

方法

• Soft X-ray absorption spectroscopy (XAS)
• Ligand-field model calculations (single cluster and double cluster)
• Charge sum rule analysis
• soft X-ray absorption spectroscopy (XAS)
• ligand-field calculations (single and double cluster)
• charge sum rule analysis
• topotactic reduction
• molecular beam epitaxy (MBE)
• PLD

关键词

• hole doping
• self doping
• oxygen non stoichiometry
• charge transfer
• orbital polarization
• hole concentration
• ni 3d hole count
• mott hubbard
• intermediate phases

亮点

• Our results challenge previous findings regarding the doping range in which superconductivity occurs in infinite-layer nickelates.
• The possibility to observe superconductivity at higher hole doping in nickelates does not necessarily contradict the analogy to cuprates.
• The oxygen K-edge spectra indicate the presence of oxygen 2p holes even in the most reduced films.
• The charge sum rule analysis challenges the nominal doping range of the superconducting dome in infinite-layer nickelates.
• The hole doping mechanism is more complex than simple hole injection, involving self-doping and oxygen non-stoichiometry.

结论

• None of the samples exhibit a pure d9 electronic configuration.
• Even in the most reduced films, the average number of Ni 3d holes remains 1.35, and superconducting samples have even higher values.
• Changes in O K-edge spectra indicate the presence of O 2p holes even in the most reduced films, suggesting a complex interplay of self-doping effects and oxygen non-stoichiometry.
• None of our samples exhibit a pure d9 configuration.
• Even when films are maximally reduced, the averaged number of nickel 3d holes is 1.35.
• Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping.
• The results suggest a complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry.

主要论断

• None of the PrNiO_x samples exhibit a pure Ni d9 configuration; even maximally reduced films have an average Ni 3d hole count of 1.35.
  • 证据: Abstract: 'none of our samples exhibit a pure d9 configuration… even when films are maximally reduced, the averaged number of nickel holes is 1.35'
• Superconducting samples have higher hole counts than previously assumed doping limit, suggesting complex interplay of self-doping and oxygen non-stoichiometry.
  • 证据: Abstract: 'Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping'
• None of the samples at any intermediate reduction stage exhibit a pure d⁹ configuration; even maximally reduced films have an average of 1.35Ni 3d holes.
  • 证据: abstract: 'none of our samples exhibit a pure d⁹ configuration' and 'the averaged number of nickel 3d holes is 1.35',full_text Fig. 3: integral values converted to nh ≈ 1.35 for most reduced sample
• Superconducting samples have even higher hole concentrations than the most reduced films, challenging the previously assumed hole-doping limit.
  • 证据: abstract: 'Superconducting samples have even higher values, calling into question the previously assumed limit of hole doping',full_text: superconducting PLD-2 sample has nh ≈ 1.55, higher than most reduced MBE-2 sample
• Oxygen K-edge spectra show that oxygen 2p holes persist even in the most reduced films, indicating finite hybridization with Ni states.
  • 证据: abstract: 'Concomitant changes in the oxygen K-edge absorption spectra upon reduction indicate the presence of oxygen 2p holes',full_text Fig. 6 shows spectral weight changes even at highest reduction
• The hole doping mechanism in infinite-layer nickelates is complex, arising from both self-doping effects and oxygen non-stoichiometry.
  • 证据: abstract: 'complex interplay of hole doping mechanisms resulting from self-doping effects and oxygen non-stoichiometry',full_text: 'three scenarios: cation non-stoichiometry, oxygen off-stoichiometry, and self-doping'

研究流程

• Sample preparation and reduction
  • 材料: PrNiO_x thin films on NdGaO₃ and SrTiO₃ substrates
  • 方法: Molecular beam epitaxy (MBE); Pulsed laser deposition (PLD); Topotactic reduction with CaH₂
  • 观察: Samples show different Ni-Ni distances and crystallinity
• X-ray absorption spectroscopy — Complex hole doping from self-doping and oxygen non-stoichiometry
  • 材料: PrNiO_x films at different reduction stages
  • 方法: Soft X-ray absorption at Ni L-edge and O K-edge; Linear polarization dependence
  • 观察: None of the samples exhibit pure d9 configuration; Even most reduced films have Ni holes ≈1.35
• Model calculations — Deviation from ideal oxygen stoichiometry (O₂.0) explains discrepancies
  • 材料: XAS spectra
  • 方法: Single-cluster and double-cluster ligand-field model calculations; Charge sum rule analysis
  • 观察: Superconducting samples have higher hole counts than previously assumed limit
• sample_preparation — Stepwise reduction enables tracking of electronic structure evolution.
  • 材料: PrNiO_x thin films on NdGaO₃ (110) and SrTiO₃
  • 方法: molecular beam epitaxy (MBE); pulsed laser deposition (PLD); topotactic reduction with CaH₂
  • 观察: Samples span perovskite to infinite-layer phases
• XAS_measurements — XAS reveals changes in electronic configuration.
  • 材料: PrNiO_x films at various reduction stages
  • 方法: Ni L-edge and O K-edge X-ray absorption spectroscopy (XAS); linear polarized X-rays; total electron yield and partial fluorescence yield
  • 观察: Evolution of spectral shape and dichroism with reduction; Shift of Ni L-edge to lower energies
• model_calculations — Calculations quantify hole concentration and orbital polarization.
  • 材料: single and double cluster ligand-field models
  • 方法: Quanty software; charge sum rule analysis
  • 观察: Simulated spectra reproduce experimental features