Anisotropic Electronic Correlations in the Spin Density Wave State of La$_3$Ni$_2$O$_7$

The bilayer nickelate superconductor La$_3$Ni$_2$O$_7$ undergoes a density wave transition near 150 K that has attracted intensive scrutiny, yet its microscopic origin remains elusive. Here we report polarization-resolved electronic Raman scattering measurements on high-quality single crystals of La$_3$Ni$2$O$7$. Below 150,K, we observe a pronounced, symmetry-dependent redistribution of spectral weight in B${1g}$ and B${2g}$ channels, consistent with the formation of spin-density-wave (SDW) gaps. Quantitative analysis reveals momentum-selective SDW gap amplitudes, with intermediate-to-strong coupling near X/Y points of the Brillouin zone and weaker coupling along the diagonal direction, indicating an unconventional SDW driven by anisotropic electronic correlations. Our results establish the electronic character of the SDW in La$_3$Ni$_2$O$_7$, and provide a microscopic foundation for understanding the emergence of high-temperature superconductivity under pressure in nickelates.

💡 Research Summary

The authors investigate the nature of the density‑wave transition that occurs near 150 K in La₃Ni₂O₇, a bilayer nickelate that becomes a high‑temperature superconductor under pressure. Using polarization‑resolved electronic Raman scattering on high‑quality single crystals, they separate the Raman response into A₁g, B₁g and B₂g symmetry channels, which, according to the Raman vertices (x² + y², x² − y² and xy), probe distinct regions of the Brillouin zone: the B₁g channel is sensitive to electronic states around the X/Y points (π, 0 and 0, π), while B₂g emphasizes the diagonal direction (π/2, π/2).

Below the transition temperature the Raman spectra show a pronounced, symmetry‑dependent redistribution of electronic spectral weight. In the B₁g channel a relatively symmetric peak appears between 600 and 720 cm⁻¹ (≈37–40 meV), whereas the B₂g channel exhibits an asymmetric peak with a high‑energy tail centered near 370 cm⁻¹ (≈23 meV). No comparable changes are observed in the A₁g channel.

To quantify the gaps the authors first remove phonon contributions (fitted with Voigt profiles) and model the background electronic scattering with a memory‑function formalism (dynamic scattering rate Γ(Ω) and mass enhancement λ(Ω)). The B₁g peak is fitted with a Lorentzian, appropriate for a relatively incoherent response, while the B₂g peak is described by the Tsuneto‑Maki (TM) function, which captures the 2Δ singularity characteristic of a density‑wave gap. The fits yield gap magnitudes Δ_B1g ≈ 37.5–40.4 meV and Δ_B2g ≈ 23 meV. Expressed as ratios to the transition temperature, 2Δ_B1g/k_BT_SDW ≈ 5.5–5.9 and 2Δ_B2g/k_BT_SDW ≈ 3.4, indicating intermediate‑to‑strong coupling at the X/Y points and weaker coupling along the diagonal.

Integrated spectral weight analyses further support this picture. The total electronic Raman weight shows a cusp‑like temperature dependence in both symmetries, with a clear kink at T_SDW ≈ 150 K. When the isolated SDW contribution is integrated, the B₁g weight first rises then falls with increasing temperature, while the B₂g weight drops sharply at the transition. This asymmetry reflects a momentum‑selective reconstruction of the electronic structure: strong, anisotropic correlations open larger gaps where the nesting is favorable (X/Y), whereas the diagonal direction experiences only modest gap opening.

The Raman‑derived gaps are consistent with previous optical conductivity and ultrafast pump‑probe measurements, yet they differ from ARPES results that reported no clear gap. The authors argue that Raman scattering, probing q ≈ 0 particle‑hole excitations with symmetry‑selected vertices, can detect “hidden” gaps that are invisible to ARPES due to matrix‑element effects or limited momentum resolution.

By comparing weak‑coupling (Fermi‑surface nesting) and strong‑coupling (local moment, Hund’s‑driven) scenarios, the authors conclude that the observed anisotropic gap structure cannot be explained solely by nesting. Instead, strong electronic correlations, which broaden the spectral function and produce incoherent particle‑hole mixing, are essential. The coexistence of strong coupling at X/Y and weaker coupling along the diagonal points to a highly anisotropic electronic correlation landscape that drives the SDW order.

Finally, the work highlights the relevance of this anisotropic SDW state to the pressure‑induced superconductivity of La₃Ni₂O₇. The momentum‑selective reconstruction may set the stage for pairing interactions once the SDW is suppressed under pressure, suggesting that spin fluctuations tied to the same anisotropic correlations could mediate the high‑temperature superconductivity. The study establishes electronic Raman scattering as a powerful probe of symmetry‑resolved electronic gaps in correlated materials and opens avenues for future Raman investigations of pressure‑tuned electronic phases in nickelates.