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The crystal-field (CF) splitting of the ⁶H_5/2 Hund's rule ground state of Sm³⁺ in the strongly correlated topological insulator SmB₆ [1,2] has been determined with high resolution resonant inelastic x-ray scattering (RIXS) at the Sm M₅ edge [3]. The valence selectivity of RIXS allows to isolate the crystal-field-split excited multiplets of the Sm³⁺(4f⁵) configuration from those of Sm²⁺(4f⁶) in intermediate valent SmB₆. We find that the quartet G₈ ground state [4] and the doublet G₇ excited state are split by D_CF=20±10meV. Considering this as an upper limit for the 4f bandwidth suggests an extremely large mass renormalization from the band structure value, which can be linked to the small coefficient of fractional parentage for the hopping of the 4f electrons [5]. The tiny band width complies with the small value of the gap and may be used to put constraints to the energies of the topological surface states.

[1] M. Dzero et al, PRL 104, 106408 (2010)
[2] T. Takimoto, J. Phys. Soc. Jpn. 80, 123710 (2011)
[3] A. Amorese et al, arXiv:1901.10808v2
[4] M. Sundermann et al, PRL 120, 016402 (2018)
[5] G. Sawatzky & R. Green, edt. E. Pavarini & E. Koch (FZ Julich), 1.1 (2016), in Quantum Materials: Experiments and Theory