Charge fluctuation in Sm₃Te₄

Ultrasonic measurements for a valence-fluctuation compound Sm₃Te₄ have been performed. Considerable dispersion around 120 K due to a thermal hopping motion of 4f-electron among Sm²⁺ and Sm³⁺ has been observed in the ultrasonic attenuation. The relaxation time of the charge fluctuation 2πτ becomes infinite at low temperatures following as τ = τ₀exp(E/k_B T), here 2πτ₀ = 2.5×10^-13 s and E = 0.136 eV. The ln T dependence of the elastic constants similar to a two-level system of amorphous glass compounds is observed at low temperatures below 15 K. These results indicate a two-level system of the 4f-electron tunneling in the random potential of Sm²⁺ and Sm³⁺ ions due to the charge glass state.