Microscopic Evidence of Charge- and Spin-Density Waves in La₃Ni₂O₇–δ Revealed by 139La-NQR

Summary

The recent discovery of superconductivity in La₃Ni₂O₇–δ with a transition temperature T_c close to 80 K at high pressures has attracted significant attention, due particularly to a possible density wave (DW) transition occurring near the superconducting dome. Identifying the type of DW order is crucial for understanding the origin of superconductivity in this system. However, owing to the presence of La₄Ni₃O₁₀ and other intergrowth phases in La₃Ni₂O₇–δ samples, extracting the intrinsic information from the La₃Ni₂O₇ phase is challenging. In this study, we employed 139La nuclear quadrupole resonance (NQR) measurements to eliminate the influence of other structural phases in the sample and obtain microscopic insights into the DW transition in La₃Ni₂O₇–δ. Below the DW transition temperature TDW ∼ 153 K, we observe a distinct splitting in the ±5/2 ↔ ±7/2 transition of the NQR resonance peak at the La(2) site, while only a line broadening is seen in the ±3/2 ↔ ±5/2 transition peak. Through further analysis of the spectra, we show that the line splitting is due to a unidirectional charge modulation. A magnetic line broadening is also observed below TDW, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state. Our results suggest a simultaneous formation of charge- and spin-density wave orders in La₃Ni₂O₇–δ, thereby offering critical insights into the electronic correlations in Ni-based superconductors.

Materials

• La₃Ni₂O₇-δ

Methods

• 139La nuclear quadrupole resonance (NQR)

Keywords

• charge density wave
• spin density wave
• unidirectional charge modulation
• magnetic ordering

Highlights

• Microscopic evidence of charge- and spin-density waves in La₃Ni₂O₇–δ revealed by 139La-NQR.
• Critical insights into electronic correlations in Ni-based superconductors.

Conclusions

• Below the DW transition temperature TDW ∼ 153 K, a distinct splitting in the ±5/2 ↔ ±7/2 transition of the NQR resonance peak at the La(2) site is observed, while only a line broadening is seen in the ±3/2 ↔ ±5/2 transition peak.
• The line splitting is due to a unidirectional charge modulation.
• A magnetic line broadening is also observed below TDW, accompanied by a large enhancement of the spin-lattice relaxation rate, indicating the formation of magnetically ordered moments in the DW state.
• Results suggest a simultaneous formation of charge- and spin-density wave orders in La₃Ni₂O₇–δ.

Main claims

• A unidirectional commensurate charge density wave (CDW) exists below T_DW ≈ 153 K in La₃Ni₂O₇-δ.
  • Evidence: NQR line splitting at La(2) site: Δν ratio between transitions matches charge modulation,Two-peak structure indicates two types of La(2) sites
• A spin density wave (SDW) forms simultaneously below the same T_DW.
  • Evidence: Magnetic line broadening appears below T_DW,1/T₁ strongly enhanced near T_DW, characteristic of spin fluctuations

Workflow

• sample_synthesis — Oxygen-deficient sample prepared.
  • Materials: La₃Ni₂O₇-δ polycrystalline sample
  • Methods: sol-gel method
  • Observations: Sample size ≈600 mg for NQR
• nqr_measurements — Line splitting indicates charge modulation (CDW).
  • Materials: La₃Ni₂O₇-δ polycrystalline powder
  • Methods: 139La nuclear quadrupole resonance (NQR) spectroscopy; Frequency-swept spin-echo; Saturation-recovery for spin-lattice relaxation time T₁
  • Observations: NQR spectrum shows La(2) site peaks at 10–19 MHz; At₁₈₈ K, four peaks identified: La₄₃₁₀, La₃₂₇-i, La₄₃₁₀-i, La₃₂₇; Below T_DW ≈153 K, ±5/2↔±7/2 transition splits; ±3/2↔±5/2 broadens
• data_analysis — CDW and SDW coexist; line splitting from CDW; magnetic broadening and 1/T₁ enhancement from SDW.
  • Materials: NQR spectra and relaxation data
  • Methods: Gaussian fitting of peaks; Extraction of splitting Δν; Analysis of magnetic broadening via FWHM decomposition; Temperature dependence of 1/T₁
  • Observations: Δν for ±5/2↔±7/2 is ≈2 times larger than for ±3/2↔±5/2; Magnetic broadening w_mag appears below T_DW; 1/T₁ increases by two orders of magnitude near T_DW