The surprising effect of electron correlations on exchange interactions and spin excitations in magnetic 2D van der Waals materials*

Despite serious effort, the nature of the magnetic interactions and the role of electron correlations in magnetic two-dimensional (2D) van der Waals (vdW) materials remain elusive. Using CrI₃ as a model system, we show that the calculated electronic structure including nonlocal electron correlations yields spin excitations consistent with inelastic neutron scattering measurements. Remarkably, this approach identifies a novel correlation-enhanced interlayer super-superexchange, which rotates the magnon Dirac lines off, and introduces a gap along, the Γ-K-M path. Our discovery provides a different perspective on the gap opening mechanism observed in CrI₃, which was previously associated with spin-orbit coupling through the Dzyaloshinskii-Moriya interaction or Kitaev interaction. Our observation elucidates new concepts to describe spin ordering and dynamics in magnetic vdW materials and demonstrates the necessity of explicit treatment of electron correlation in the broad family of 2D magnetic materials.