Two dimensional magnets with mixed Frenkel-Wannier characters and their qualitative deviations from Sugano-Tanabe diagrams of atomic-multiplets

Transitions that are atom- (or ion) local, or possibly a molecular ligand around a magnetic transition metal ion are traditionally described by Sugnao-Tanabe diagrams which have been used for nearly the past eight decades to phenomenologically assign molecular term symbols to such localized onsite dd transitions. In stark contrast, the solid state community has built a description of excitons in analogy with the hydrogen atom and its attendant Rydberg series to describe extended excitonic states in semiconductors, called Wannier-Mott excitons. My ab-initio calculations reveal that the 2D magnets can host both kinds of excitons. I will show materials examples where our ab-initio approach will explore excitons in qualitative deviations from molecular-orbital theory and I will reveal the shortcomings of Ligand-field approaches in describing energies and brightness of excitons in a range of 2D and 3D magnets.