Rattling in Clathrate Compounds of ROs4Sb12 and R3Pd20Ge6

We have performed ultrasonic experiments on clathrate compounds ROs 4 Sb 12 and R 3 Pd 20 Ge 6 to explore rattling and tunneling of a guest rattler's motion. We have found frequency dependence of the elastic constants and ultrasonic attenuations, which is called as ultrasonic dispersion, due to thermally activated rattling. At low temperatures, Curie-type elastic softening obeying 1/ T with antiferro-type intersite interaction, which corresponds to quantum tunneling, appears and continues even to the lowest 20 mK in LaOs 4 Sb 12 and La 3 Pd 20 Ge 6 . The understanding of the local charge fluctuation, rattling and tunneling, being coupled to conduction electrons in metals is a basic issue in the aspect of electron–phonon interaction in strongly correlated phenomena such as heavy fermion state, superconductivity and multi-channel Kondo screening effect.

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