A RIXS investigation of the crystal-field splitting of Sm${}^{3+}$ in SmB${}_6$

Abstract

The crystal-field (CF) splitting of the ${}^6{H}_{5/2}$ Hund's rule ground state of Sm${}^{3+}$ in the strongly correlated topological insulator SmB${}_6$ has been determined with high resolution resonant inelastic x-ray scattering (RIXS) at the Sm M${}_5$ edge. The valence selectivity of RIXS allows isolating the crystal-field-split excited multiplets of the Sm${}^{3+}$ (4$f^5$) configuration from those of Sm${}^{2+}$ (4$f^6$) in intermediate valent SmB${}_6$. The very large energy range of RIXS allows the crystal-field analysis of a high lying multiplet at about 2.4\,eV that has the same total angular momentum $J$ as the ground state so that ambiguities due to the elastic tail can be avoided. We find that the $\hat{I}{“}_7$ doublet and $\hat{I}{“}_8$ quartet of the ${}^6{H}_{5/2}$ Hund's rule ground state are split by $\hat{I}{”}_{{}^6{H}_{5/2}}^{CF}$\,=\,20$\pm$10\,meV which sets an upper limit for the 4$f$ band width. This indicates an extremely large mass renormalization from the band structure value, pointing out the need to consider the coefficients of fractional parentage for the hopping of the 4$f$ electrons.

8 pages, 6 figures