Logarithmic temperature dependence of samarium ion valence in the heavy-fermion $\mathrm{S}{\mathrm{m}}_x\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{O}{\mathrm{s}}_4\mathrm{S}{\mathrm{b}}_{12}$

We have measured x-ray absorption spectra at the $\mathrm{Sm}{L}_3$ edge to investigate the Sm-ion valence of $\left(\mathrm{S}{\mathrm{m}}_x\mathrm{L}{\mathrm{a}}_{1-x}\right)\mathrm{O}{\mathrm{s}}_4\mathrm{S}{\mathrm{b}}_{12}$, in which field-insensitive heavy-fermion behavior appears at low temperatures for $x=1$. It has been found that the Sm-ion valance shifts to $2+$ with La ion substitution; from $v=+2.78(x=1)$ to $v=+2.73(x=0.2)$ at 10 K. For all $x$ investigated, its temperature dependence shows a $logT$ behavior, indicating that the valence change is caused by “an unconventional Kondo effect” associated with $\mathrm{Sm}4f$-electron charge degrees of freedom. Almost $x$ independence of “the associated Kondo temperature” $({\tilde{T}}_{\mathrm{K}}=56\pm 10\mathrm{K})$ indicates that the Kondo effect has a local nature, attributable to the cage structure of the filled skutterudite.

Figure 1

XAS spectra at the $\mathrm{Sm}{L}_3$ edge of $\left(\mathrm{S}{\mathrm{m}}_{0.2}\mathrm{L}{\mathrm{a}}_{0.8}\right)\mathrm{O}{\mathrm{s}}_4\mathrm{S}{\mathrm{b}}_{12}$ measured at $T=60\mathrm{K}$. The green, blue, and red lines indicate the $\mathrm{S}{\mathrm{m}}^{3+}, \mathrm{S}{\mathrm{m}}^{2+}$, and total components fitted to the experimental data (closed circles), respectively.

Figure 2

(a) Temperature dependence of XAS spectra around the $\mathrm{Sm}{L}_3$ edge for $x=0.2$. (b) Sm concentration dependence of XAS spectra measured at 10 K.

Figure 3

Temperature dependence of the average Sm-ion valence, $v\left(T\right)$, in $\left(\mathrm{S}{\mathrm{m}}_x\mathrm{L}{\mathrm{a}}_{1-x}\right)\mathrm{O}{\mathrm{s}}_4\mathrm{S}{\mathrm{b}}_{12}$. $v\left(T\right)$ shows a $logT$-like behavior in the middle temperature range. The solid lines are best fits of Eq. (3) to the data.

Figure 4

$T/{\tilde{T}}_{\mathrm{K}}$ dependence of the normalized Sm-ion valence $\left[v\left(T\right)-v\left(0\right)\right]/\left[v\left(\infty \right)-v\left(0\right)\right]$ of $\left(\mathrm{S}{\mathrm{m}}_x\mathrm{L}{\mathrm{a}}_{1-x}\right)\mathrm{O}{\mathrm{s}}_4\mathrm{S}{\mathrm{b}}_{12}$. The solid line, which can be approximated as $0.50+0.55{log}_{10}(T/{\tilde{T}}_{\mathrm{K}})-0.18[{log}_{10}(T/{\tilde{T}}_{\mathrm{K}}){]}^3+\cdots$, corresponds to the fitting curve of Eq. (3).

Figure 5

Sm concentration dependencies of “the associated Kondo temperature” ${\tilde{T}}_{\mathrm{K}}$ and the Sm-ion valence.