Role of Valence Fluctuations in the Superconductivity of Ce122 Compounds

Pressure dependence of the Ce valence in ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$ has been measured up to 24 GPa at 300 K and to 17 GPa at 18–20 K using x-ray absorption spectroscopy in the partial fluorescence yield. A smooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in ${\mathrm{Ce}({\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x)}_2{\mathrm{Si}}_2$ at 20 K. A very weak, monotonic increase of the valence with $x$ was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results show no evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.

Figure 1

(a) Schematic view of pressure-temperature phase diagram of ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ [3]. The pressure for ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ is shifted by 10 GPa (upper horizontal axis) to be consistent with that of ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$. The superconducting region consists of SC I and SC II. Antiferromagnetic (AFM) order is only observed in ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$. (b) The pressure-temperature phase diagram of ${\mathrm{Ce}({\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x)}_2{\mathrm{Si}}_2$ as a function of $x$ [7]. Kondo temperature ($T_K$) is also shown for comparison.

Figure 2

Temperature dependences of PFY-XAS for (a) ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$ and (b) ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. (c) Examples of the fits for ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$ and ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. (d) Derived ratios for the PFY-XAS spectral weight of the compounds as a function of temperature.

Figure 3

PFY-XAS spectra of ${\mathrm{CeCu}}_2{\mathrm{Ge}}_2$ as a function of pressure at (a) 300 and (b) 16–18 K. (c) Estimated spectral weight ratio from the fit to the PFY-XAS spectra as a function of pressure at 300 K (closed circle) and 16–18 K (open circle) with a schematic figure of two superconducting regions.

Figure 4

(a) PFY-XAS spectra of ${\mathrm{Ce}({\mathrm{Cu}}_{1-x}{\mathrm{Ni}}_x)}_2{\mathrm{Si}}_2$ as a function of $x$ at 20 K. (b) Ce valence (spectral weight ratio) estimated from the fit to the PFY-XAS spectra with a schematic figure of two superconducting regions. (c) Pressure-induced change in the Ce valence for ${\mathrm{Ce}({\mathrm{Cu}}_{0.95}{\mathrm{Ni}}_{0.05})}_2{\mathrm{Si}}_2$ with the data of ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$ from the literature [27, 28].