Novel Magnetism and Superconductivity on Filled Skutterudite Compounds which Possess Rattling

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Ultrasonic measurements on the Filled-Skutterudite compounds Pr_<1-x>NdxOs_4Sb_<12>(x=0.45, 0.25, 1.0)have been performed. Frequency dependences of elastic constant C11 were investigated with longitudinal ultrasonic frequencies varying between 34 to 245 MHz on these samples. A characteristic ultrasonic dispersion, which had also been observed in PrOs_4Sb_<12> (x=0) at around 30K, is also observed in the dilute systems and shifts gradually to higher temperature with increasing the Nd concentration x. On the other hand, the C11 of x=0.45 sample shows softening and levels off feature at T=0.25K. This result implies that the Nd3+ ion's degenerate CEF ground state is split due to some order even in the Pr^<3+> ion's contribution to SC. In addition, a magnetically robust elastic softening of C11 was found in the end material NdOs_4Sb_<12> (x=1.0) under magnetic field 2 T<H<10T at low temperature T<1K, which suggest a presence of the off-center tunneling as same as LaOs_4Sb_<12> and PrOs_4Sb_<12>. Above results imply that an origin of the heavy-fermion state of NdOs_4Sb_<12> could possibly be related to a degenerate quantum ground state due to the off-center degrees of freedom.