Spectroscopic evidence of competing diagonal spin interactions and spin disproportionation in the bilayer nickelate La 3 Ni 2 O 7

A comprehensive spectroscopic map of the electronic, magnetic, and lattice excitations is presented for the bilayer nickelate La₃Ni₂O₇ using Raman scattering at ambient pressure. Upon entering the spin density wave state below 153 K, the A1g channel exhibits an abrupt electronic spectral gap with a clear isosbestic point. In contrast, the B1g and B2g channels are dominated by pronounced two-magnon (2M) excitations, representing an unambiguous signature of incipient Mottness. These 2M signals in both channels constitute direct evidence for two distinct in-plane spin exchange interactions along the Ni-O bonding and its diagonal directions. Calculations based on the spin wave theory further reveal that the 2M mode in the B2g channel arises from the competition between two bond-diagonal antiferromagnetic interactions mediated by nickel dx²-y² orbitals. Furthermore, emergent low-energy 2M excitations below 10 meV are found to originate from distinct, weaker spin moments, strongly supporting spin disproportionation. Simultaneously, an anomalous softening of B1g phonons from 280 down to 4.5 K is uncovered, suggesting the presence of an incipient lattice instability leading to checkerboard-type breathing modulations. Collectively, these findings identify a ground state of the bilayer nickelate characterized by competing bond-diagonal interactions, spin disproportionation, and an incipient lattice instability, establishing key ingredients for understanding the mechanism of nickelate superconductivity.