We are focusing on investigating ultrasonic dispersion (UD) due to rattling in the R
= La, Pr, Nd, Sm) system in order to examine a possible relationship between the unconventional superconductivity in PrOs4
 or magnetically robust large residual entropy in low temperature specific heat in SmOs4
 and its local charge degrees of freedom due to anharmonic potential . Intriguingly, NdOs4
show double UD with different temperature and time scales, while non-4f
shows only a single UD. Most recently, we found that PrOs4
also show double UD. The another dispersion occurs at around Tc
~1.8 K, where the activation energy of the lower UD is estimated a one digit smaller value of ~5 K than the values obtained 67 K for NdOs4
and 105 K for SmOs4
By monitoring a systematic change of the UD among the Sb-based filled skutterudites, we also find that both higher and lower activation energies E1
increase and corresponding attempt times τ0, (1)
, τ0, (2)
decrease when the rare-earth ion changes from R
= La to Sm in ROs4
. When the rare-earth ion changes from R
= La to Sm, not only the guest ion mass but also the amount of free space for the rattler in the atomic cage increases, owing to the lanhanide contraction, i.e
., the ionic radius of the guest ion decreases, and relatively rigid character of the atomic cage of Sb-based filled skutterudites. We speculate that such changes in the guest ion conditions give rise to an additional anharmonicity of the potential,e.g
., flat base or double wells, for the local ionic configuration, and result in the emergence of additional UD in 4f
-electron system, R
= Pr, Nd and Sm.
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, J. Phys. Soc. Jpn. 74 (2005) 246.
 T. Yanagisawa et al.
, J. Phys. Soc. Jpn. 80 (2011) 043601.
 W. M. Yuhasz et al.
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 Y. Aoki et al.
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For more information please see our paper: T. Yanagisawa et al.
J. Phys. Soc. Jpn. 80 (2011) 043601.
(also available on cond-mat/1010.1387)