Racah Materials: Role of Atomic Multiplets and Intermediate Valence in f-Electron Systems

The electronic structure of PuB6, an actinide analog of SmB6 , was investigated making use of a combination of the density functional theory (DFT), and the exact diagonalization (ED) of an effective discrete Anderson impurity model. Intermediate valence ground state with the f-shell occupation n4f =5.5 for the Pu atom in PuB6 is calculated. The 5f-shell magnetic moment is completely compensated by the moment carried by the electrons in the conduction band. Already in DFT, PuB6 is an insulator with a small amount of holes near the X-point, and the indirect band gap of ≈60 meV. This band gap becomes direct in DFT+ED calculations supporting the idea of “topological Kondo insulator” in PuB6. Connection between the electronic structure of PuB6 and δ-Pu is established. We propose that these materials belong to a new class of intermediate valence “Racah” materials with the multi-orbital “Kondo-like” singlet ground-state.