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.
Off-center rattling of La-ion in the filled skutterudite compound LaOs4Sb12 has been investigated by means of ultrasonic measurements. An elastic constant C 11 shows a remarkable frequency dependence in elastic constant and ultrasonic attenuation at around 25 K. This frequency dependent ultrasonic dispersion reveals thermally activated Γ23-type off-center rattling of La-ion. An attempt time τ0 = 5.0×10−11 sec and an activation energy E = 127 K were obtained from the analysis.KEYWORDS: filled skutterudite, elastic constant, ultrasonic attenuation, off-center, rattlingIn recent years, the filled skutterudites with the formula RT 4 X 12 (R = rare-earth, T = transition-metal, X = pnictgen) have been energetically studied because these systems exhibit a wide variety of strongly correlated phenomena.1) Especially, a finding of the off-center rattling and tunneling in the first Pr-based heavy fermion superconductor PrOs 4 Sb 12 by ultrasonic measurements attracts much interest.2) It is understood that the rattling motion of the rare-earth ion, which is filled in an oversized (OsSb 3 ) 4 cage, is a thermally activated excitation of ionic quantum states, which correspond to anharmonic potential in the cage. On the other hand, a quantum tunneling motion is understood as local charge fluctuations through the potential barrier. In ultrasonic measurements, the former displays ultrasonic dispersion (UD) and the later exhibits low-temperature elastic softening. An UD due to the rattling motion of Pr-ion is observed in Γ 23 -symmetry related ultrasonic mode in heavy fermion superconductor PrOs 4 Sb 12 . Interestingly, an elastic softening of PrOs 4 Sb 12 stops at the superconducting transition point T C = 1.85 K, which suggests that off-center degrees of freedom contribute to the heavy fermion superconductivity.LaOs 4 Sb 12 has the LaFe 4 P 12 -type BCC structure with space group (Im3) and lattice constant a = 9.308Å. 3)This compound becomes a conventional BCS superconductor below T C ∼ 0.74 K and possesses an electronic specific heat coefficient γ ∼ 36-56 mJ/mol K 2 . 4) Nuclear spin-lattice relaxation time 1/T 1 shows a coherence peak just at T C , followed by exponential dependence with a gap size of 2∆/k B T C ∼ 2.3 at low temperatures.5) These results indicate that a conventional character of the superconducting state in this compound. To gain the better understanding of a relation between the off-center motion and the heavy fermion superconductor, elastic constants C 11 and C 44 , and ultrasonic attenuation coefficient α 11 were measured on the filled skutterudite compound LaOs 4 Sb 12 . Rectangular parallelepiped shaped single crystal specimens with a dimension of 0.55× 0.63× 1.64 mm (#1) and 0.47× 0.71× 1.76 mm (#2) with the [001] axis aligned in the ultrasound propagation were used for these measurements. The elastic constant C = * E-mail address: f06a005a@mail.cc.niigata-u.ac.jp ρv 2 was estimated by assuming a mass density ρ = 9.721 g/cm 3 . The ultrasonic wave was generated and detected by piezoelectric LiNbO 3 tran...
Ultrasonic investigation of a single crystal of the filled skutterudite antiferromagnetic compound PrOs4As12 was performed. Curie-type softening appeared in CL111 = CB + C44, is explained in terms of the quadrupole susceptibility modified by a crystalline electric field with a Γ (2) 4 -triplet ground state and a Γ1-singlet first excited state. The quadrupole susceptibility analysis finds that the intersite quadrupole interactions in this compound are negative, indicating an antiferro-quadrupolar type interaction. Additional ultrasonic features, appeared in an ordered phase under magnetic fields between 2 T and 3 T for H [111] Characterization of the rare earth filled skutterudite arsenides RT 4 As 12 (R = rare-earth, T = Fe, Ru, Os) have recently progressed, with successful synthesis of the single crystals of the arsenides.1, 2 Unlike the phosphide and antimonide Pr-based filled skutterudites, the arsenides have not been investigated in great detail. In view of the interesting examples of strongly correlated electron behavior found in the Pr-based filled skutterudite phosphides and antimonides, 3-6 we embarked on an investigation of the Pr-based filled skutterudite arsenides PrOs 4 As 12 by ultrasonic method. Single crystals of PrOs 4 As 12 were grown by means of mineralization process with Cd based flux at high temperatures and pressures. Specific heat measurements on the single crystals of PrOs 4 As 12 show two successive transitions at T 1 = 2.2 K (antiferro-magnetic (AFM) phase III) and T 2 = 2.3 K (unknown phase II), which is observed as a sharp peak and a relatively broad peak, respectively. An enhanced electronic specific heat coefficient γ ∼ 1 J/mol K 2 was observed in the phase III. By applying magnetic fields H ∼ 3 − 4 T, upper phase II were suppressed.1, 2 Analysis of magnetic susceptibility and electrical resistivity and specific heat indicate the Pr 3+ ion having a triplet crystalline electric field (CEF) ground state. Since the Pr 3+ triplet ground state in T h symmetry contains orbital degrees of freedom such as electric quadrupoles, we have performed ultrasonic measurements on single crystals of PrOs 4 As 12 in order to investigate the involvement of orbital degrees of freedom to the upper phase II.The orientation of crystal with respect to applied magnetic field was determined by using x-ray Laue diffractions. The LiNbO 3 transducers with dimension of 1.3 mm × 1.3 mm × 0.2 mm were bonded on parallel surfaces of the sample for the generation and detection of the ultrasound. The sound frequency of 3rd overtone with frequencies of 52-58 MHz was selected to balance an impedance matching of the components and directivity of the ultrasound. A 3 He-evapolation refrigerator was used for the low-temperature measurements down to 0.4 K with a superconducting magnet up to 10 T.Temperature dependence of longitudinal and transverse elastic constants C L111 and C T 111 is shown in Figs. 1 (a) and (b), respectively. Left axes in Figs. 1 display change relative to the 80-K elastic constants. On the upper right...
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