Ultrasonic study of the charge-fluctuation compoundSm3Te4

Abstract

We have measured the ultrasonic velocity and attenuation coefficient of the rare-earth chalcogenide ${\mathrm{Sm}}_{3}{\mathrm{Te}}_{4}$ to examine the valence-fluctuation effect due to the coexistence of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions in the ratio of 1:2. The ultrasonic dispersion around 120 K indicates that the charge-fluctuation time obeys the activation-type temperature dependence $\ensuremath{\tau}={\ensuremath{\tau}}_{0}\mathrm{exp}{(E/k}_{B}T)$ with a characteristic time $2\ensuremath{\pi}{\ensuremath{\tau}}_{0}=2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}\mathrm{sec}$ and an activation energy $E=0.136\mathrm{eV}.$ The absence of the phase transition due to the charge ordering in ${\mathrm{Sm}}_{3}{\mathrm{Te}}_{4}$ means a freezing of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions in random distribution at low temperatures. We find an elastic softening with $\mathrm{ln}T$ dependence below 15 K down to a spin glass transition around 1.3 K. This striking behavior is attributed to a two-level system due to the tunneling of $4f$ electrons among randomly distributed ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions. This result is similar to the ionic tunneling in amorphous glass compounds. Employing the group-theoretical analysis, we show some aspects of charge-fluctuation modes in ${\mathrm{Sm}}_{3}{\mathrm{Te}}_{4}$ and a possible mechanism for the charge glass state of ${\mathrm{Sm}}^{2+}$ and ${\mathrm{Sm}}^{3+}$ ions.