It was recently proposed that the intermediate valent Kondo insulator SmB₆ [1,2] is a topological insulator. The proposal is appealing since rare earth Kondo insulators have the necessary ingredients for strong spin-orbit coupling and electrons of opposite parity (4f and 5d). The surface topology depends directly on the symmetries of the bulk states involved. Knowledge about the crystal electric field ground state is therefore crucial. For example, theoretical predictions for the spin texture of the sought-after topological surface states differ for the 4f⁵ J_5/2 ground state configuration being the G₈ quartet or the G₇ doublet: e.g. the winding direction is opposite [3-6].

We performed bulk-sensitive, core-level non-resonant inelastic hard-x ray scattering (NIXS) measurements that target specifically the ground state symmetry of SmB₆. Here we make use of the highly excitonic character of the higher multipole transitions made possible by the large momentum transfer q in our experiment and of the strong directional dependence of the scattering function S(q,w) [7-9], giving the symmetry information of the ground state wave function, even for cubic compounds. We found from our spectra that the G₈ quartet governs the ground state symmetry and hence the topological properties of SmB₆ [10]. Our experimental results contradict the predictions of band structure calculations [11].

References:
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[7] T. Willers, L.H. Tjeng et al., PRL109, 046401 (2012)
[8] M. Sundermann, L.H. Tjeng et al., Sci. Rep. 5, 17937 (2015)
[9] M. Sundermann, L.H. Tjeng et al., PNAS 113, 13989 (2016)
[10] M. Sundermann, L.H. Tjeng et al., arXiv: 1706.08072
[11] C.-J. Kang et al., J. Phys. Soc. Jpn 84, 024722 (2015)