Daily Overview: Today’s highlight focuses on advancing the understanding of the electronic structure of hybrid Ruddlesden-Popper nickelates. Using Raman scattering spectroscopy, the research team comprehensively characterized the electronic, magnetic, and lattice excitations of the bilayer nickelate La₃Ni₂O₇ under ambient pressure. For the first time, they simultaneously observed two types of in-plane spin exchange interactions along the Ni-O bond direction and the diagonal direction in the spin-density-wave state, revealing the bond-diagonal antiferromagnetic competition mediated by the nickel d_{x²-y²} orbitals. Moreover, the discovery of low-energy two-magnon excitations provides direct spectroscopic evidence for spin disproportionation, while anomalous phonon softening hints at an incipient lattice instability. These results collectively establish the correlation among competitive spin interactions, spin disproportionation, and lattice response in this system, providing key experimental foundations for understanding the microscopic mechanism of nickelate superconductivity. arXiv submission processing window: 2026-02-05 06:39 to 2026-02-05 06:39 UTC.

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

  • Relevance Score: 5.3525
  • Authors: Dong-Hyeon Gim, Dirk Wulferding, Hengyuan Zhang, Meng Wang, Kee Hoon Kim
  • Affiliations: Sun Yat-Sen University, Seoul National University, Sejong University
  • Link: http://arxiv.org/abs/2602.05365v1

Summary: Using Raman scattering spectroscopy, this study comprehensively characterizes the electronic, magnetic, and lattice excitations of the bilayer nickelate La₃Ni₂O₇ under ambient pressure. Upon entering the spin density wave state below 153 K, a sharp electronic gap accompanied by a clear isosbestic point appears in the A₁g channel, while the B₁g and B₂g channels exhibit pronounced two-magnon (2M) excitations, a distinct signature of a quasi-Mott insulator. The coexistence of 2M signals in both channels directly demonstrates two distinct in-plane spin exchange interactions along the Ni–O bond direction and its diagonal direction. Spin wave calculations further reveal that the 2M mode in the B₂g channel originates from the competition between two bond-diagonal antiferromagnetic interactions mediated by the nickel d_{x²-y²} orbitals. Moreover, the low-energy 2M excitation below 10 meV arises from weaker, different spin moments, strongly supporting spin disproportionation. Concurrently, an anomalous softening of B₁g phonons from 280 K down to 4.5 K is observed, indicating an incipient lattice instability that may lead to a checkerboard breathing modulation. These results collectively establish that the ground state of this bilayer nickelate features competing bond-diagonal interactions, spin disproportionation, and incipient lattice instability, providing key ingredients for understanding the mechanism of superconductivity in nickelates.