SmB₆ is a mysterious compound that is electrically insulating but yet it exhibits quantum oscillations, which are a telltale signature of the metallic state. Adding to the enigma is the possibility that SmB₆ is a topological Kondo insulator. Here, we report first-principles, parameter-free all-electron electronic-structure calculations on SmB₆, which yield the band structure and crystal-field splittings within the f -electron complex in accord with experiments. Predicted energies of several magnetic phases where charge, spin and lattice degrees of freedom are treated on an equal footing are found to be extremely close, indicating the key role of spin fluctuations in SmB₆. Our analysis shows that the topological Kondo state of SmB₆ is robust regardless of its magnetic configuration. However, it lies near the metal-insulator transition where the Fermi surface derived from the predicted ground state explains the experimentally observed bulk quantum oscillations. The calculated effective electron mass at the Fermi surface and specific heat further explain how the material is essentially insulating in agreement with the experimental results.

References:
R. Zhang, et al., arXiv:2003.11052
M. Hartstein, et al., Nat. Phys. 14, 166–172 (2018)