Breakdown of the strong multiplet description of the Sm2+ ion in the topological Kondo insulator SmB6: specific heat studies

We have theoretically confirmed the existence of in-gap real quantum-mechanical states in SmB 6 , which have been suggested by experiments. These in-gap states, below the hybridization gap of 20 meV, are related to the Sm 2+ ion states and can be revealed by calculations within the spin-orbital | LSL z S z 〉 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 7 F (4 f 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 SmB 6 , confirming the consistency and adequateness of our theoretical approach with the breakdown of the strong multiplet description of the Sm 2+ ion in SmB 6 .