Ultrasonic dispersion due to off-center rattling in NdOs4Sb12NdOs4Sb12

Ultrasonic investigation for the single crystals of ferromagnetic filled skutterudite compound NdOs4Sb12NdOs4Sb12 was performed. A marked frequency dependence of the ultrasonic dispersion has been observed in a longitudinal elastic constant C11C11 at temperature centered around 45 K; however, no dispersion was measured in the transverse elastic constant C44C44. Such a clear mode difference in the ultrasonic dispersion, which has been found in PrOs4Sb12PrOs4Sb12, strongly suggests that Nd ion in NdOs4Sb12NdOs4Sb12 bears thermally activated off-center rattling effect.