Abstract
We have theoretically confirmed the existence of in-gap real quantum-mechanical states in SmB6, which have been suggested by experiments. These in-gap states, below the hybridization gap of 20 meV, are related to the Sm2+ ion states and can be revealed by calculations within the spin-orbital |LSLzSz〉 space, with L = 3 and S = 3. Our approach overcomes difficulties related to the singlet J = 0 multiplet ground state. The in-gap states originate from the 49-fold degenerated term 7F (4f 6), which is split by cubic crystal-field (CEF) and spin-orbit (s − o) interactions. There is competition between these interactions: the six-order CEF interactions produce a 7-fold degenerated ground state, whereas the s − o interactions, even the weakest one, produce a singlet (J = 0) ground state. We have found preliminary CEF and s − o parameters that produce the lowest states at 0 K (singlet) and 91 K (triplet) and the next triplet at 221 K, i.e., within the hybridization gap. The derived states well explain the large extra specific heat of SmB6, confirming the consistency and adequateness of our theoretical approach with the breakdown of the strong multiplet description of the Sm2+ ion in SmB6.
Highlights
|LSLzSz〉 space, with L = 3 and S = 3
Studied for more than 50-years, the compound SmB61–4 has recently become a strong candidate for a 3-dimensional topological Kondo insulator (TKI) with a robust bulk insulating gap[5,6,7,8,9,10,11,12,13]
We have found preliminary values of λs−o = +110 K with cubic CEF B60 = 0.1 K and B64 = −2.1 K interactions that well reproduce the experimental temperature dependence of the samarium contribution to the specific heat of SmB6
Summary
The experimentally derived specific heat of SmB6 exhibits a large extra heat compared with that of isostructural, nonmagnetic LaB6; see Fig. 1, which was redrawn from refs[21,22]. The Ni2+ and Co2+ ions are 3d8 and 3d7 quantum systems, respectively, and are both characterized by L = 3, similar to the 4f6 (Sm2+) quantum system In this contribution, we performed calculations of the fine electronic structure of the Sm2+ (4f6) ion in SmB6 within the spin-orbital |LSLzSz〉 space for the 7F term (L = 3 and S = 3) given by the two Hund’s rules. Leaving the problem of understanding of other than specific-heat phenomena for future studies, we conclude that our theoretical approach, with the spin-orbit coupling assumed to have a finite, relatively weak value, reveals a fine electronic structure with a quite large number of low-energy states
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