We are focusing on investigating ultrasonic dispersion (UD) due to rattling in the ROs₄Sb₁₂ (R = La, Pr, Nd, Sm) system in order to examine a possible relationship between the unconventional superconductivity in PrOs₄Sb₁₂ [1] or magnetically robust large residual entropy in low temperature specific heat in SmOs₄Sb₁₂ [2] and its local charge degrees of freedom due to anharmonic potential [1]. Intriguingly, NdOs₄Sb₁₂ and SmOs₄Sb₁₂ show double UD with different temperature and time scales, while non-4f system LaOs₄Sb₁₂ shows only a single UD. Most recently, we found that PrOs₄Sb₁₂ also show double UD. The another dispersion occurs at around T_c ~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 NdOs₄Sb₁₂ and 105 K for SmOs₄Sb₁₂.
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 and E₂ increase and corresponding attempt times τ_{0, (1)}, τ_{0, (2)} decrease when the rare-earth ion changes from R = La to Sm in ROs₄Sb₁₂. 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.

[1] E. D. Bauer et al., Phys. Rev. B 65 (2002) 100506(R).
[2] S. Sanada et al., J. Phys. Soc. Jpn. 74 (2005) 246.
[3] T. Yanagisawa et al., J. Phys. Soc. Jpn. 80 (2011) 043601.
[4] W. M. Yuhasz et al., Phys. Rev. B 71 (2005) 104402.
[5] Y. Aoki et al., Physica B 378-380 (2006) 54.

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)

重い電子系化合物URu₂Si₂の圧力下電気抵抗測定によって作られた温度ー圧力相図です。本物質は常圧においてT = 17.5 Kで、秩序変数・秩序波数共に不明の「隠れた秩序」を示す物質です。同研究室の日高助教がインデンター型圧力セルを用いて測定しました。電気抵抗の絶対値をグラデーションで表現すると、綺麗なオーロラのようなコントラストの中に隠れた秩序相が浮かび上がります。隠れた秩序相は静水圧下（P ≧ 1 GPa)で一次転移を伴って反強磁性相に転移することがわかっていますが、対応する領域で相境界線の勾配が変化し、相境界線が若干ブロードになっていることが相図からみてとれます。
もっと詳しく知りたい方は、強相関電子磁性１研究室の研究トピックス「URu₂Si₂ における隠れた秩序と微弱反強磁性」 をご覧下さい。