Oxygen-isotope effect on density wave transitions in La₃Ni₂O₇

摘要

本研究通过氧同位素替换（¹⁶O→¹⁸O），利用电阻率和μ子自旋旋转（μSR）实验，系统探索了双层Ruddlesden-Popper镍酸盐La₃Ni₂O₇中电荷密度波（CDW）和自旋密度波（SDW）转变的同位素效应。电阻率测量显示，CDW转变温度在¹⁸O取代后显著升高约6 K，而μSR结果则表明SDW转变温度在实验误差范围内不受影响。拉曼光谱证实了同位素替换的有效性及晶格声子模的软化。这一对比鲜明的同位素响应表明，晶格振动（即电子-声子耦合）在CDW序的形成中扮演重要角色，而SDW序主要源于电子相互作用。研究结果揭示了两种密度波序的不同微观起源，并提示电子-声子耦合可能对Ruddlesden-Popper镍酸盐的超导配对机制具有潜在关联，为理论模型提供了关键约束。

材料

• La₃Ni₂O₇ polycrystalline

方法

• resistivity measurements
• muon-spin rotation/relaxation (μSR)
• Raman spectroscopy
• X-ray diffraction
• thermogravimetric analysis
• oxygen-isotope substitution

关键词

• oxygen isotope effect
• cdw transition
• sdw transition
• electron phonon coupling
• isotope shift
• lattice vibrations
• charge order
• spin order

亮点

• The isotope shift of TCDW is approximately 6 K.
• Raman spectroscopy confirms effective isotope substitution and softening of lattice phonon modes.
• The contrasting isotope responses highlight the different microscopic origins of CDW and SDW orders in La₃Ni₂O₇.

结论

• A clear isotope effect is observed in the CDW transition: the transition temperature (TCDW) increases upon 18O substitution.
• In contrast, the SDW transition temperature remains unaffected within experimental uncertainty.
• These findings point to a strong involvement of lattice vibrations in the formation of charge order, while spin order appears to be predominantly of electronic origin.

主要论断

• A clear oxygen isotope effect is observed on the CDW transition: T_CDW increases by ≈6 K upon substitution of ¹⁶O with ¹⁸O.
  • 证据: abstract: 'A clear isotope effect is observed in the CDW transition: T_CDW increases upon ¹⁸O substitution',full_text Fig. 2b shows ΔT_CDW ≈ 6 K derived from derivative curves
• The SDW transition temperature is unaffected by isotope substitution within experimental uncertainty (ΔT_SDW ≈ 0 K).
  • 证据: abstract: 'the SDW transition temperature remains unaffected within experimental uncertainty',full_text Fig. 3b: T_SDW = 148.3 K for ¹⁶O and 148.6 K for ¹⁸O, well within error (Table 1)
• Lattice vibrations (electron-phonon coupling) play a strong role in CDW formation, while SDW is predominantly electronic in origin.
  • 证据: abstract: 'strong involvement of lattice vibrations in the formation of charge order, while spin order appears to be predominantly of electronic origin',full_text: 'these findings point to a strong involvement of lattice vibrations in the formation of charge order'
• The electron-phonon coupling manifested in the CDW response may be relevant to the superconducting pairing mechanism in Ruddlesden-Popper nickelates.
  • 证据: abstract: 'electron-phonon coupling, manifested through the CDW response to isotope substitution, may be relevant to the superconducting pairing mechanism',full_text: 'The results suggest that electron-phonon coupling… may be relevant to the superconducting pairing mechanism'

研究流程

• sample_preparation_and_isotope_substitution — Isotope substitution achieved without structural or stoichiometric change.
  • 材料: La₃Ni₂O₇ polycrystals
  • 方法: oxygen isotope substitution (¹⁶O→¹⁸O) via annealing; mass spectrometry for ¹⁸O content (82%)
  • 观察: Crystal structure unchanged by isotope substitution; Oxygen content nearly stoichiometric
• phonon_characterization — Raman confirms effective and uniform isotope distribution.
  • 材料: ¹⁶O and ¹⁸O substituted La₃Ni₂O₇
  • 方法: Raman spectroscopy at room temperature
  • 观察: Three oxygen-dominated modes show frequency softening; Oxygen participation parameter γ_O close to 1 for these modes
• density_wave_transition_measurements — Isotope effect: CDW couples to lattice, SDW is electronic in origin.
  • 材料: ¹⁶O and ¹⁸O La₃Ni₂O₇
  • 方法: resistivity for CDW transition; muon spin rotation (μSR) for SDW transition
  • 观察: T_CDW increases by ≈6 K for ¹⁸O sample; T_SDW unchanged within uncertainty