Ultrasonic dispersion due to off-center rattling in NdOs4Sb12

Abstract Ultrasonic investigation for the single crystals of ferromagnetic filled skutterudite compound NdOs 4 Sb 12 was performed. A marked frequency dependence of the ultrasonic dispersion has been observed in a longitudinal elastic constant C 11 at temperature centered around 45 K; however, no dispersion was measured in the transverse elastic constant C 44 . Such a clear mode difference in the ultrasonic dispersion, which has been found in PrOs 4 Sb 12 , strongly suggests that Nd ion in NdOs 4 Sb 12 bears thermally activated off-center rattling effect.