Rattling in clathrate compounds of ROs₄Sb₁₂ and R₃Pd₂₀Ge₆

We have performed ultrasonic experiments on clathrate compounds ROs ₄Sb₁₂ and R₃Pd₂₀Ge₆ 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 LaOs₄Sb₁₂ and La₃Pd₂₀Ge₆. 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.