Sm-based heavy-fermion compound ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$ has been investigated by soft x-ray ($h\nu =1070–1600\mathrm{eV}$) and hard x-ray (HX; $h\nu =7932\mathrm{eV}$) spectroscopy. The HX photoemission spectroscopy clearly demonstrates that the strongly mixed-valence state and the heavy-fermion state coexist in the bulk. It is found that the Sm valence decreases below 100 K, indicating that the Kondo coherence develops with approaching the proposed Kondo temperature. Our theoretical analyses suggest that the origin of the coexistence in ${\mathrm{SmOs}}_4{\mathrm{Sb}}_{12}$ is the coincidence of two conditions, namely, (i) the energy difference between Sm divalent and trivalent states is very small and (ii) the hybridization between Sm $4f$ and conduction electrons is weak.

• Received 24 February 2006

DOI:https://doi.org/10.1103/PhysRevLett.98.156402