Magnetic-field-independent ultrasonic dispersions in the magnetically robust heavy fermion system SmOs4Sb12

Elastic properties of the filled skutterudite compound SmO _s4Sb₁₂ have been investigated by ultrasonic measurements. The elastic constant C11(ω) shows two ultrasonic dispersions at ∼15 and ∼53K for frequencies ω 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 = 105K and E₁ = 409 K, respectively. The latter, E₁, is the highest value reported thus far in the Sb-based filled skutterudites. The presence of magnetically robust ultrasonic dispersions in SmO_s4Sb₁₂ 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.