Oxygen-isotope effect on density wave transitions in La₃Ni₂O₇

Summary

This study systematically explores the isotope effects on the charge density wave (CDW) and spin density wave (SDW) transitions in the bilayer Ruddlesden-Popper nickelate La₃Ni₂O₇ through oxygen isotope substitution (¹⁶O→¹⁸O) using resistivity and muon spin rotation (μSR) experiments. Resistivity measurements reveal a significant increase in the CDW transition temperature by approximately 6 K after ¹⁸O substitution, while μSR results indicate that the SDW transition temperature remains unaffected within experimental error. Raman spectroscopy confirms the effectiveness of the isotope substitution and the softening of lattice phonon modes. This contrasting isotope response suggests that lattice vibrations, i.e., electron-phonon coupling, play a crucial role in the formation of the CDW order, whereas the SDW order primarily originates from electronic interactions. The findings unveil distinct microscopic origins of the two density wave orders and hint at the potential relevance of electron-phonon coupling to the superconducting pairing mechanism in Ruddlesden-Popper nickelates, providing key constraints for theoretical models.

Materials

• La₃Ni₂O₇ polycrystalline

Methods

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

Keywords

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

Highlights

• 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.

Conclusions

• 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.

Main claims

• A clear oxygen isotope effect is observed on the CDW transition: T_CDW increases by ≈6 K upon substitution of ¹⁶O with ¹⁸O.
  • Evidence: 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).
  • Evidence: 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.
  • Evidence: 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.
  • Evidence: 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'

Workflow

• sample_preparation_and_isotope_substitution — Isotope substitution achieved without structural or stoichiometric change.
  • Materials: La₃Ni₂O₇ polycrystals
  • Methods: oxygen isotope substitution (¹⁶O→¹⁸O) via annealing; mass spectrometry for ¹⁸O content (82%)
  • Observations: Crystal structure unchanged by isotope substitution; Oxygen content nearly stoichiometric
• phonon_characterization — Raman confirms effective and uniform isotope distribution.
  • Materials: ¹⁶O and ¹⁸O substituted La₃Ni₂O₇
  • Methods: Raman spectroscopy at room temperature
  • Observations: 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.
  • Materials: ¹⁶O and ¹⁸O La₃Ni₂O₇
  • Methods: resistivity for CDW transition; muon spin rotation (μSR) for SDW transition
  • Observations: T_CDW increases by ≈6 K for ¹⁸O sample; T_SDW unchanged within uncertainty