Ultrasonic dispersion and off-center rattling in heavy fermion superconductor PrOs4Sb12

Ultrasonic attenuation measurements have been firstly performed for a large single crystal of PrOs 4 Sb 12 with the dimensions of 5.97 × 0.6 × 0.6 mm 3 . Remarkable frequency dependence around 20–40 K has been observed in the elastic constant and attenuation coefficient of the longitudinal C 11 mode associated with E g symmetry strain in part, which results from a thermally activated off-center rattling with E g symmetry of a Pr ion inside a Sb icosahedron cage. Parameters of a characteristic time τ 0 = 3.1 × 10 - 11 s and an activation energy E = 225 K were obtained. This E g rattling involving a local charge fluctuation inside a Sb cage periodically arranged may couple to the conduction electrons. As a result, the electron–phonon coupling would lead to heavy fermion and its superconductivity in PrOs 4 Sb 12 .