Racah materials: role of atomic multiplets in intermediate valence systems

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Chemistry

Background:

• The electronic properties of intermediate valence compounds like SmB6 remain a long-standing mystery.
• Understanding the origin of the nonmagnetic insulating state in SmB6 is key to exploring novel quantum phenomena.

Purpose of the Study:

• To elucidate the electronic structure and ground state of SmB6.
• To investigate the role of electron correlations in forming the insulating gap.
• To explore potential actinide analogs and classify new classes of materials.

Main Methods:

• Combined local density approximation (LDA) and exact diagonalization (ED) of an Anderson impurity model.
• Calculated f-shell occupation and energy levels of the ground and excited states.
• Analyzed the impact of electron correlations on the band gap and electronic structure.

Main Results:

• Identified an intermediate valence ground state for Sm in SmB6 with f-shell occupation of 5.6.
• The ground state is a singlet, with the first excited triplet state approximately 3 meV higher.
• LDA predicts a narrow band insulator; electron correlations enhance the gap, making it indirect, which is crucial for topological Kondo insulator behavior.
• An intermediate valence singlet ground state was also found for Pu in PuB6.

Conclusions:

• SmB6 exhibits a singlet ground state, explaining its nonmagnetic insulating nature.
• Many-body effects are essential for the indirect band gap formation in SmB6, supporting its topological Kondo insulator classification.
• [Sm, Pu]B6 represent a new class of intermediate valence materials with multi-orbital Kondo-like singlet ground states.