Daily Overview: The highlighted work today focuses on an ambient-pressure Raman scattering study of the electronic, magnetic, and lattice excitations in the bilayer nickelate La₃Ni₂O₇. Experimental observations show that after entering the spin-density-wave state below 153 K, the A₁g channel exhibits an abrupt electronic spectral gap and an isosbestic point at about 50 meV, whereas the B₁g and B₂g channels are dominated by pronounced two-magnon excitations, indicating two in-plane spin exchange interactions along the Ni–O bonds and their diagonal directions. Calculations based on spin-wave theory further indicate that the B₂g two-magnon mode originates from competition between two bond-diagonal antiferromagnetic interactions mediated by Ni d_{x²−y²} orbitals, while low-energy two-magnon excitations below 10 meV arise from weaker spin moments and support spin disproportionation. The study also observes anomalous softening of the B₁g phonon from 280 K to 4.5 K, suggesting an incipient lattice instability that leads to checkerboard-like breathing modulation. These results link competitive bond-diagonal interactions, spin disproportionation, and lattice instability, providing key elements for understanding the superconducting mechanism of nickelates. arXiv submission processing window: 2026-11-04 00:00 to 2026-11-04 00:00 UTC.

1. Spectroscopic Evidence of Competing Diagonal Spin Interactions and Spin Disproportionation in the Bilayer Nickelate La$_3$Ni$_2$O$_7$

Summary: This study used an ambient-pressure Raman scattering system to measure the electronic, magnetic, and lattice excitations of the bilayer nickelate La₃Ni₂O₇. Upon entering the spin-density-wave state below 153 K, the A₁g channel exhibits an abrupt electronic spectral gap and an isosbestic point at about 50 meV, whereas the B₁g and B₂g channels are dominated by pronounced two-magnon excitations, indicating two in-plane spin exchange interactions along the Ni–O bond and its diagonal direction. Calculations based on spin-wave theory further indicate that the B₂g two-magnon mode originates from competition between two bond-diagonal antiferromagnetic interactions mediated by the Ni d_{x²−y²} orbitals; the low-energy two-magnon excitations below 10 meV arise from weaker spin moments, supporting spin disproportionation. The study also finds anomalous softening of the B₁g phonon from 280 K to 4.5 K, suggesting an incipient lattice instability that leads to checkerboard-like breathing modulation. These results link competing bond-diagonal interactions, spin disproportionation, and lattice instability, providing key elements for understanding the superconducting mechanism of nickelates.