Contrasting Momentum-Selective Spin-Density-Wave Gaps in Bilayer and Trilayer Nickelates

Resolving where the density-wave gap opens in momentum space is essential for identifying the microscopic origin of the instability in layered nickelates. Using polarization-resolved electronic Raman scattering, we map the momentum selectivity of the spin-density-wave (SDW) gap in trilayer La4Ni3O10. We observe a SDW-induced redistribution of spectral weight on both the $α$ pocket at the Brillouin-zone centre and a portion of the $β$ pocket near the zone boundary, characterized by gap energies of approximately 55~meV. In contrast, no comparable spectral weight suppression is observed along the diagonal region of $β$ pockets, implying little or no gap opening. This gap topology contrasts sharply with that in La3Ni2O7, where anisotropic SDW gaps open solely on the $β$ pocket. Our results establish a distinct momentum-space gap topology between bilayer and trilayer nickelates, placing new constraints on the ordering wave vector and the mechanism of the density-wave instability relevant to superconductivity.

💡 Research Summary

The authors investigate the momentum‑space structure of the spin‑density‑wave (SDW) gap in the trilayer nickelate La₄Ni₃O₁₀ and compare it with the previously studied bilayer compound La₃Ni₂O₇. Using polarization‑resolved electronic Raman scattering, they decompose the Raman response into three symmetry channels—A₁g, B₁g and B₂g—each probing distinct regions of the Brillouin zone (BZ). The A₁g channel is sensitive to excitations near the BZ centre (Γ point) and corner (M point), the B₁g channel to the zone‑boundary X/Y points, and the B₂g channel to the diagonal Γ‑M direction.

Temperature‑dependent Raman spectra reveal a pronounced loss of electronic spectral weight below ~800 cm⁻¹ (≈55 meV) in both the A₁g and B₁g channels when cooling through the density‑wave transition temperature T_DW ≈ 140 K. This loss appears as a dip‑hump feature, characteristic of a gap opening, and indicates that the SDW gap opens on the α pocket at the BZ centre and on a portion of the β pocket near the X/Y points. In stark contrast, the B₂g channel shows virtually no change across T_DW, implying that the diagonal portion of the β pocket remains ungapped.

These observations differ sharply from the bilayer La₃Ni₂O₇, where earlier Raman work showed an anisotropic SDW gap that opens exclusively on the β pocket (both B₁g and B₂g channels display clear gap signatures) while the α pocket stays metallic. The gap magnitude in La₃Ni₂O₇ is smaller (≈37 meV in B₁g, ≈23 meV in B₂g).

The authors argue that the lack of zone‑folded phonon modes or a sharp amplitude mode in the Raman spectra disfavors a lattice‑driven charge‑density‑wave (CDW) scenario. Instead, the data support an electronically driven density‑wave with a dominant spin component. Screening effects, evaluated with a tight‑binding model, reduce the Raman intensity of the gap excitations by roughly 40 % in La₄Ni₃O₁₀ and 50 % in La₃Ni₂O₇, but screening alone cannot explain the complete absence of an A₁g gap feature in La₃Ni₂O₇, confirming that the α pocket does not host a gap there.

From the symmetry‑resolved Raman response, the authors infer the ordering wave vector. In La₄Ni₃O₁₀ the data are consistent with a vector Q₂ that connects the α pocket to the β pocket near the X/Y points (approximately (0, 0.61, 0) in the 1‑Ni Brillouin zone). This explains the simultaneous gap opening on both pockets while leaving the diagonal β region untouched. In La₃Ni₂O₇, the appropriate vector is Q₁, which nests parallel segments of the β pocket (approximately (0, 0.5, 0) in the 2‑Ni Brillouin zone), accounting for the gap on the entire β sheet.

The study thus establishes a distinct momentum‑space gap topology between bilayer and trilayer nickelates: trilayer La₄Ni₃O₁₀ exhibits a partially gapped Fermi surface involving both α and β pockets with weak coherence, whereas bilayer La₃Ni₂O₇ shows a strongly anisotropic, coherent gap confined to the β pocket. This difference provides new constraints on theoretical models of the density‑wave instability and its relationship to superconductivity in layered nickelates. The authors suggest that future neutron scattering, high‑resolution ARPES, and advanced strong‑coupling calculations are needed to fully resolve the competing nesting vectors and to clarify how the SDW order influences the markedly different superconducting transition temperatures observed in these materials.