Breakdown of the strong multiplet description of the Sm ion in the topological Kondo insulator SmB: specific heat studies.

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