New Spectroscopy Solves an Old Puzzle: The Kondo Scale in Heavy Fermions

Resonant inelastic x-ray scattering (RIXS) yields clear evidence of spectroscopic Kondo scales in heavy fermions. In ${\mathrm{YbInCu}}_4$ and ${\mathrm{YbAgCu}}_4$ RIXS probes the ${\mathrm{Yb}}^{2+}$ component of the hybrid ground state and the temperature dependence of the Yb $4f$ occupation. We report a sudden valence change at a phase transition in ${\mathrm{YbInCu}}_4$, but a continuous temperature dependence in ${\mathrm{YbAgCu}}_4$, consistent with the predictions of the Anderson impurity model, for a Kondo temperature $T_K=70\mathrm{K}$. These results solve a long-standing controversy and establish RIXS as a quantitative probe of the electronic structure of strongly correlated electron systems.