Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically Robust Heavy Fermion System SmOs4Sb12

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Elastic properties of the filled skutterudite compound SmOs₄Sb₁₂ have been investigated by ultrasonic measurements. The elastic constant C₁₁(ω) shows two ultrasonic dispersions at 15 K and 53 K 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 = 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.