Goto Terutaka

Elastic Properties of Filled Skutterudite Compounds

Elastic properties have been investigated on the skutterudite compounds: PrFe 4 P 12 , NdFe 4 P 12 and CeRu 4 Sb 12 . A pronounced softening behavior towards low temperatures was observed in elastic constants of NdFe 4 P 12 in zero field. On the other hand, it was not observed in either compounds of PrFe 4 P 12 and CeRu 4 Sb 12 in zero field. However, a remarkable softening behavior in c 11 of PrFe 4 P 12 towards low temperatures appeared in applying magnetic fields. All these behaviors will be discussed in terms of the effect due to the crystalline electric field (CEF) split levels of the ground-state J multiplet.

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 rattlers 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.

Off-Center Rattling and Tunneling in Filled Skutterudite LaOs4Sb12

Off-center rattling of La-ion in the filled skutterudite compound LaOs 4 Sb 12 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 Γ 23 -type off-center rattling of La-ion. An attempt time τ 0 = 5.0×10 -11 sec and an activation energy E = 127 K were obtained from the analysis.

Anisotropy in the Magnetic Phase Diagrams of CexLa1-xB6

Anisotropy in the magnetic phase diagrams ( H - T plane) of Ce x La 1- x B 6 and the Ce concentration dependence of the magnetic phase diagrams are presented. Phase boundaries of phase IV in the diagrams are isotropic. Present results suggest the absence of quadrupolar degeneracy in phase IV. Field induced antiferromagnetism is shown in Ce x La 1- x B 6 .

Guest Ion Motion in Cage Structure Crystals Investigated by Raman Scattering

Last five years we have mainly investigated phonons in the following cage compounds by Raman scattering: RT 4 X 12 , A 8 Ga 16 Ge 30 , La 3 Pd 20 Ge 6 , RB 6 , KOs 2 O 6 . From this systematic investigation we have clarified the following dynamical properties of the guest ion trapped in the large cages. The vibration of the guest ion is highly anharmonic due to weak atomic interaction between the guest ion and cage atom. This anharmonicity universally decreases the vibration energy of the guest ion with decreasing temperature. The existence of carriers decreases the interaction between the guest atoms and changes the dispersion curve of the guest ion mode to flat, which is the experimental evidence of the weak interaction between the guest atoms. For thermoelectricity, it is found in A 8 Ga 16 Ge 30 that the off-center rattling decreases lattice thermal conductivity.

Quadrupole Susceptibility of HoB2C2

We have investigated the elastic constants of the ternary rare earth compound HoB 2 C 2 with a tetragonal structure by means of the ultrasonic method. The elastic constants of ( C 11 - C 12 )/2, C 44 and C 66 , corresponding to the transverse ultrasonic modes exhibit the softening proportional to reciprocal temperature below about 40 K. This softening is commonly enhanced in the ordered phase IV below T C 1 =5.9 K. An abrupt increase of elastic constants is found in the ordered phase III below T C 2 =5.0 K characterized by the simultaneous antiferro-type ordering of electric quadrupole and magnetic dipole moments. The quadrupole susceptibility based on a model consisting of a ground state E-doublet and an excited singlet with a low energy in HoB 2 C 2 is employed for the analysis of the softening above T C 1 =5.9 K. The enhancement of the elastic softening in the phase IV of HoB 2 C 2 resembles to the soft C 44 mode in the phase IV in Ce 0.75 La 0.25 B 6 .

Elastic Properties of TbCoGa5 under Magnetic Field

The elastic properties of TbCoGa 5 single crystal were investigated by the measurement of the elastic constants under magnetic fields. The results of the measurement are consistent with successive component-separated magnetic orderings in TbCoGa 5 . The elastic properties in high magnetic fields indicate that the spin system is strongly affected by the quadrupole moment system in TbCoGa 5 . Such a mutual influence assumes an important role in the occurrence of the successive component-separated magnetic transitions.

Thermal Expansion Measurements on Ce0.75La0.25B6

We have made low-temperature thermal expansion and magnetostriction measurements on the Kondo compound Ce 0.75 La 0.25 B 6 . With decreasing temperature, Ce 0.75 La 0.25 B 6 in phase IV expands along the [001] axis and shrinks along the [111] axis. A tiny anisotropic distortion is found in this phase. Probably, some kinds of macroscopic symmetry are breaking in phase IV.

Magnetic Field Dependence of Elastic Constants in Pr1-xCaxMnO3

We have investigated charge and orbital (quadrupolar) ordering in perovskite manganites Pr 1- x Ca x MnO 3 ( x =0.35, 0.40, 0.50) by ultrasonic measurements. Only transverse ( C 11 - C 12 )/2 mode of the x =0.35, 0.40, and 0.50 compounds shows a softening from 400 K down to the charge ordering point T co =220-240 K. This softening is originated from the bilinear coupling of the quadrupole moment of the e g orbital in Mn 3+ ion to the elastic strain. Furthermore, the elastic softening in both ( C 11 - C 12 )/2 and C 44 modes due to the coupling of the charge fluctuation mode to elastic strain is observed just above T co , especially for the x =0.50 compound. The considerable magnetic field dependence of the elastic constants is obseved. In particular, the magnetic fields up to 12 T releases completely the elastic softening of ( C 11 - C 12 )/2 mode due to the fact that the charge fluctuation is suppressed under 12 T in the x =0.40 compound.

Coulombic Insight into Charge Ordering in Yb4As3 and Sm3Te4

Charge ordering in Yb 4 As 3 and Sm 3 Te 4 , complementary f -electron compounds with the same crystalline structure, is investigated on the basis of a Coulomb lattice gas model; the Coulomb interactions between ions are fully taken into account within a classical treatment. The Madelung energy of the system at finite temperatures is calculated with a Monte Carlo (MC) method simulating the Boltzmann distribution of multivalent cations on equivalent lattice sites. The unit cell for the MC simulations is extended to contain 32 molecular formulas in it. The model is successful in distinguishing the two mixed-valence compounds with different characteristics of charge ordering. Yb 4 As 3 shows a phase transition (Verwey transition) due to charge ordering while Sm 3 Te 4 continuously freezes into a glassy state on cooling.