Truly bulk-sensitive spectroscopic measurements of valence in heavy fermion materials

Intermediate valence is one of the typical phenomena of systems with strong electronic correlation. The Anderson impurity model predicts a scaling of the valence with the reduced temperature T/T(K), which is difficult to observe by traditional surface-sensitive electronic spectroscopies. This paper presents results obtained by resonant inelastic X-ray scattering (RIXS), a bulk-sensitive configuration- and chemical-specific technique. The temperature dependence of the valence of YbInCu(4) and YbAgCu(4) was measured by tuning the incident energy to the resonance of the Yb(2+) spectral component. In the case of YbInCu(4) a sharp valence transition, as known from thermodynamical measurements, has been found. The valence of YbAgCu(4) reveals a smooth dependence consistent with a Kondo temperature T(K) = 70 K. These findings establish RIXS as a powerful tool for measuring bulk electronic properties of solids.