Abstract
We have investigated an off-center local oscillator of a guest atom in cage of clathrate compounds R 3Pd 20Ge 6 ( R = La , Ce, Pr, Nd) and a filled skutterudite PrOs 4Sb 12 using ultrasonic measurements. The observation of ultrasonic dispersions in transverse C 44 mode of R 3Pd 20Ge 6 and in transverse ( C 11 - C 12 ) / 2 mode of PrOs 4Sb 12 revealed the rattling of the off-center oscillator characterized by a relaxation time with Arrhenius-type temperature dependence. A low-temperature softening of C 44 in La 3Pd 20Ge 6 below 3 K down to 20 mK and a softening of ( C 11 - C 12 ) / 2 in PrOs 4Sb 12 above the heavy Fermion superconducting transition indicate the charge fluctuation due to the degenerate off-center oscillators.
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