The elastic properties of a filled skutterudite PrOs_4Sb_{ 12} with a heavy Fermion superconductivity at T_C=1.85 K have been investigated. The elastic softening of (C_{ 11}-C_{ 12})/2 and C_{ 44} with lowering temperature down to T_C indicates that the quadrupolar fluctuation due to the CEF state plays a role for the Cooper paring in superconducting phase of PrOs_4Sb_{ 12}. A Debye-type dispersion in the elastic constants around 30 K revealed a thermally activated Gamma_{ 23} rattling due to the off-center Pr-atom motion obeying tau=tau_{ 0}exp(E/k_{ B}T) with an attempt time tau_0=8.8*10^{ -11} sec and an activation energy E=168 K. It is remarkable that the charge fluctuation of the off-center motion with Gamma_{ 23} symmetry may mix with the quadrupolar fluctuation and enhance the elastic softening of (C_{ 11}-C_{ 12})/2 just above T_C.
Categories: cond-mat.str-el