Rattling in Clathrate Compounds of ROs₄Sb₁₂and R₃Pd₂₀Ge₆

Abstract

We have performed ultrasonic experiments on clathrate compounds ROs4Sb12 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.

Magnetic-Field-Independent Ultrasonic Dispersions in the Magnetically Robust Heavy Fermion System SmOs4Sb12

“…The ultrasonic mode dependence suggests that the UD is derived from a symmetrical off-center charge fluctuation of the guest ion, e.g., Γ 23 -symmetry off-center mode for ROs 4 Sb 12 and Γ 5 symmetry for R 3 Pd 20 Ge 6 . 12) We note that our results, the softening of C 44 in particular, are completely different from those of the previous work by Nakanishi et al 20) The major difference could simply come from the fact that higher frequency and shorter pulses, i.e., more focused ultrasonic waves, were used in the present measurements, which help to minimize the irregular reflection of ultrasonic waves from the crack or cavity on the surface or inside the crystal. A good example of this effect can be seen in a deviation of the lowest frequency (33 MHz) data of C 11 in Fig.…”

“…9,10) By ultrasonic measurements, a frequency-dependent upturn of the elastic constant with an ultrasonic attenuation peak, called an ultrasonic dispersion (UD), has been observed in the cage compounds, such as R 3 Pd 20 Ge 6 (R = La, Ce, Pr) and ROs 4 Sb 12 (R = La, Pr, Nd). [11][12][13] Moreover, an unusual decrease in elastic constant has also been observed at very low temperatures in the La-based compounds. Owing to the copresence of UD and low-temperature elastic softening in non-4 f La-based compounds, the origins of these phenomena are expected to share a common root cause, originating from a novel local charge degree of freedom.…”

Quadrupole Ordering and Rattling Motion of Clathrate Compound Pr3Pd20Ge6

“…The dispersion around 15 K in Pr 3 Pd 20 Ge 6 has already been reported in our previous work. 4) In the present experiments, we found no sign of ultrasonic dispersion in both C 11 and ðC 11 À C 12 Þ=2 modes. We proceed to analyze the frequency dependences of the elastic constant C 44 ð!Þ and of the attenuation coefficient 44 ð!Þ using the Debye-type ultrasonic dispersion formula.…”

“…[1][2][3][4] The Cr 23 C 6 -type cubic structure of R 3 Pd 20 Ge 6 with space group Fm " 3m (No. 225) has a primitive unit cell consisting of 12 R ions, 80 Pd atoms, and 24 Ge atoms, 5) where the rare-earth R ions occupy the crystallographically nonequivalent 4a and 8c sites.…”

“…The filled skutterudite PrOs 4 Sb 12 with the cage structure also shows the ultrasonic dispersion due to the rattling. 4) In order to verify the rattling in the clathrate compounds, spectroscopic measurements using Raman, 10) neutron, 11,12) and x-ray 13,14) methods have been carried out. Theoretical studies have also been conducted to explain the ultrasonic dispersion due to the rattling motions of local phonons being strongly coupled to low-lying optical phonons or conduction electrons.…”