Temperature and pressure-induced valence transitions inYbNi2Ge2andYbPd2Si2

Abstract

We have measured the temperature and pressure-induced Yb valence transitions in tetragonal ${\text{YbNi}}_{2}{\text{Ge}}_{2}$ and ${\text{YbPd}}_{2}{\text{Si}}_{2}$ using x-ray absorption spectroscopy in the partial fluorescence yield mode and resonant x-ray emission spectroscopy. A temperature dependence of the Yb valence on the order of 0.1 has been measured, consistent with the magnetic-susceptibility study. The crossover from the low-temperature state having a stronger mixed valence to a high-temperature local moment behavior is analyzed within the Anderson impurity model. Pressure-induced second-order valence transitions are observed for both compounds with a more gradual transition in ${\text{YbPd}}_{2}{\text{Si}}_{2}$ than that of ${\text{YbNi}}_{2}{\text{Ge}}_{2}$. The mean valences are slightly less than $3+$ at ambient pressure but increase with applying pressure. Small variations in the Yb valence on the order of 0.03--0.05 can result in drastic change in the physical properties such as magnetic order and transport properties. Our results show that the Yb valence is noninteger around the quantum critical point.