Elastic properties of the filled skutterudite compound SmOs₄Sb₁₂ have been investigated by ultrasonic measurements. The elastic constant C₁₂(\omega) shows two ultrasonic dispersions at ∼15 K and ∼53 K for frequencies \omega between 33 and 316 MHz, which follow a Debye-type formula with Arrhenius-type temperature-dependent relaxation times, and remain unchanged even with applied magnetic fields up to 10 T. The corresponding activation energies were estimated to be E₂ = 105 K and E₁ = 409 K, respectively. The latter, E₁, is the highest value reported so far in the Sb-based filled skutterudites. The presence of magnetically robust ultrasonic dispersions in SmOs₄Sb₁₂ implies a possibility that an emergence of a magnetically insensitive heavy fermion state in this system is associated with a novel local charge degree of freedom which causes the ultrasonic dispersion.

(also available on cond-mat/1010.1387)

Figures (a) Comparison of the ultrasonic dispersions that appear in elastic constant C₁₁ of ROs₄Sb₁₂ (R = La-Sm) at several frequencies. Lower arrowheads with numbers 1 and 2 indicate the relaxation point \omega\tau_i ∼ 1 for i = 1 and 2, respectively. (\omega is ultrasonic frequency and \tau is relaxation time) The displayed data have been shifted to eliminate overlapping with each other and the SmOs₄Sb₁₂ data are magnified three times for the \Delta C₁₁/C₁₁-axis. (b) Arrhenius plots of the characteristic parameters of the ultrasonic dispersions (Attempt time: \tau₀(i), Activation Energy: E_i) for ROs₄Sb₁₂ (R = La-Sm).