Anisotropic Electronic Correlations in the Spin Density Wave State of La₃Ni₂O₇

Summary

Using polarization-resolved electronic Raman scattering measurements on high-quality La₃Ni₂O₇ single crystals, we observe a pronounced, symmetry-dependent spectral weight redistribution across the density-wave transition below 150 K: the B₁g channel exhibits an asymmetric peak, while the B₂g channel shows a symmetric broad peak, corresponding to electronic excitations near the X/Y points of the Brillouin zone and along the diagonal directions, respectively. Quantitative analysis extracts two sets of SDW gap values, with the B₁g channel gap approximately 37.5–40.4 meV (2Δ/k_BT_c ≈ 5.5–5.9) and the B₂g channel gap about 23.0 meV (2Δ/k_BT_c ≈ 3.4), indicating intermediate-to-strong coupling for the former and weak coupling for the latter. This momentum-selective anisotropic coupling strength cannot be explained by simple weak-coupling nesting theory, revealing that the unconventional SDW originates from anisotropic electronic correlations. The temperature dependence of the gap is significantly weaker than mean-field expectations, and the isotropy of the B₂g channel along with the weak anisotropy of the B₁g channel further support the coexistence of two distinct coupling mechanisms. This work establishes the electronic characteristics of the SDW in La₃Ni₂O₇, providing a microscopic foundation for understanding the emergence of high-temperature superconductivity in nickelates under pressure.

Materials

• La₃Ni₂O₇

Methods

• polarization-resolved electronic Raman scattering
• quantitative gap analysis

Keywords

• spin density wave (sdw) gap
• anisotropic electronic correlations
• momentum selective gap
• brillouin zone x/y points

Highlights

• Establishes electronic character of SDW in La₃Ni₂O₇, providing microscopic foundation for understanding superconductivity under pressure.

Conclusions

• SDW gaps in La₃Ni₂O₇ are momentum-selective with intermediate-to-strong coupling near X/Y points and weaker along diagonal, indicating an unconventional SDW driven by anisotropic electronic correlations.

Main claims

• SDW gaps open in La₃Ni₂O₇ below 150 K with different amplitudes on different Fermi surface regions.
  • Evidence: Raman spectra show redistribution of spectral weight in B₁g and B₂g channels below 150 K; fitting yields gap values of ≈37-40 meV (B₁g) and ≈23 meV (B₂g).
• The SDW coupling is weak along the diagonal direction (B₂g) and medium-to-strong near X/Y points (B₁g).
  • Evidence: 2Δ/kT ratios: 3.4 for B₂g (weak coupling), 5.5-5.9 for B₁g (medium-to-strong coupling); lineshape asymmetry indicates strong correlation in B₁g.
• The SDW transition is primarily electronic (spin) rather than lattice-driven.
  • Evidence: No anomalies in phonon modes across transition; no zone-folded phonons; electronic origin supported by lack of lattice signatures.

Workflow

• sample_growth — Single crystals of La₃Ni₂O₇ were grown.
  • Materials: La₂O₃; NiO; O₂
  • Methods: high-pressure optical floating zone furnace; solid-state reaction
  • Observations: High-qualityLa₃Ni₂O₇ single crystals
• Raman_spectroscopy_measurements — SDW gap opening observed in B₁g and B₂g channels.
  • Materials: single crystals
  • Methods: polarization-resolved Raman scattering; confocal geometry; 532 nm laser; Stirling fridge
  • Observations: Electronic Raman response in A₁g, B₁g, B₂g channels; Spectral weight redistribution below 150 K in B₁g and B₂g; No change in A₁g
• analysis_and_fitting — Anisotropic SDW gaps with different coupling strengths on different Fermi surface regions.
  • Methods: memory function fitting; Tsuneto-Maki function; Lorentzian fitting
  • Observations: Gap values: Δ_B₁g=37.5-40.4 meV, Δ_B₂g=23.0 meV; 2Δ/kT ratios: 5.5-5.9 for B₁g, 3.4 for B₂g; Weak coupling in B₂g, medium-to-strong in B₁g