Abstract
We address the long-standing mystery of the nonmagnetic insulating state of the intermediate valence compound SmB6. Within a combination of the local density approximation (LDA) and an exact diagonalization (ED) of an effective discrete Anderson impurity model, the intermediate valence ground state with the f-shell occupation 〈n4f〉 = 5.6 is found for the Sm atom in SmB6. This ground state is a singlet, and the first excited triplet state ~3 meV higher in the energy. SmB6 is a narrow band insulator already in LDA, with the direct band gap of ~10 meV. The electron correlations increase the band gap which now becomes indirect. Thus, the many-body effects are relevant to form the indirect band gap, crucial for the idea of “topological Kondo insulator" in SmB6. Also, an actinide analog PuB6 is considered, and the intermediate valence singlet ground state is found for the Pu atom. We propose that [Sm, Pu]B6 belong to a new class of the intermediate valence materials with the multi-orbital “Kondo-like" singlet ground-state. Crucial role of complex spin-orbital f n–f n+1 multiplet structure differently hybridized with ligand states in such Racah materials is discussed.
Highlights
We apply this concept to another 4f and 5f systems, using SmB6 and PuB6 as examples of the “Racah materials”
The electronic structure calculations are performed within the density functional plus dynamical mean-field theory (“LDA+ + ”18) approach combining the local density approximation (LDA) with an exact diagonalization (ED) of the Anderson impurity model for SmB6 and PuB6
The intermediate valence singlet ground states are found for these materials
Summary
A new Green’s function GLDA(z) (which corresponds to G(z) from Eq (2) with the self energy Σ set to zero), and a new value of the 5f-shell occupation are obtained from the solution of Eq (3). For SmB6, the Slater integrals were chosen as F0 = 6.87 eV, and F2 = 9.06 eV, F4 = 6.05 eV, and F6 = 4.48 eV24 They corresponds to commonly accepted values for Coulomb U = 6.87 eV and Hund exchange J = 0.76 eV, and are in the ballpark of the parameters commonly used in the calculations of the rare-earth materials[25]. For PuB6, the Slater integrals F0 = 4.0 eV, and F2 = 7.76 eV, F4 = 5.05 eV, and F6 = 3.07 eV were chosen[26] They corresponds to commonly accepted values for Coulomb U = 4.0 eV and exchange J = 0 .64 eV.
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