Temperature dependence of the Yb valence inYbCu5andYbCu5−xAlxKondo compounds studied by x-ray spectroscopy

Abstract

We have performed bulk-sensitive measurements of the temperature dependence of the Yb valence in the hexagonal ${\text{YbCu}}_{5\ensuremath{-}x}{\text{Al}}_{x}\text{ }(0\ensuremath{\le}x\ensuremath{\le}2)$ and cubic ${\text{YbCu}}_{5}$ Kondo compounds using x-ray absorption spectroscopy in the partial fluorescence yield and resonant x-ray emission spectroscopy. A large difference in the valence is found between the cubic $(\ensuremath{\sim}2.96)$ and hexagonal $(\ensuremath{\sim}2.5)$ $x=0$ systems, which evidences a strong structural dependence of the Yb electronic properties. For all studied compounds, a temperature dependence of the valence on the order of 0.02 is measured and found consistent with the magnetic susceptibility. This crossover from the low-temperature state having a higher mixed valence to a high-temperature local-moment behavior is analyzed within the Anderson impurity model (AIM). A good agreement is obtained between our data and the AIM. Evidence is brought for valence fluctuation to occur near the quantum critical point at $x=1.5$ and near the antiferromagnetic phase transition at $x=2.0$.