Takabatake Toshiro

Well-Defined Crystal Field Splitting Schemes and Non-Kramers Doublet Ground States of f Electrons in PrT2Zn20 (T = Ir, Rh, and Ru)

Inelastic neutron scattering studies of Pr\(T_{2}\)Zn20 (\(T\) = Ir, Rh, and Ru) were carried out in order to reveal \(f\) electron states indispensable for the low-temperature properties of Pr\(T_{2}\)Zn20 associated with multipole degrees of freedom. Sharp excitation peaks, which are located within excitation energies of less than 10 meV, are attributed to crystal field splitting level schemes of \(4f^{2}\) in Pr3+ ions. Data of the Ir-based compound is explained by a non-Kramers doublet ground state in the local symmetry \(T_{d}\) for this system. Results of the Rh-based compound are explained by the scheme for the Pr-site local cubic symmetry \(T\) in the low-temperature structural phase, which is lower than \(T_{d}\). In the Ru-based compound, the doublet is split as a result of the loss of local cubic symmetry through structural transformation. These experimental results confirm the non-Kramers doublet ground state that causes the quadrupole ordering or orbital Kondo effect predicted for this series...

Universal scaling in the dynamical conductivity of heavy fermion Ce and Yb compounds

Dynamical conductivity spectra [σ(ω)] have been measured for many heavy-fermion (HF) Ce and Yb compounds. A characteristic excitation peak has been observed in the infrared region of σ(ω) for all the compounds, and has been analyzed in terms of a simple model based on conduction ( c )– f electron hybridized band. A universal scaling is found between the observed peak energies and the estimated c – f hybridization strengths of these HF compounds. This scaling demonstrates that the model of c – f hybridized band can generally and quantitatively describe the low-energy charge excitations in a wide range of HF compounds.

Universal Relation between Guest Free Space and Lattice Thermal Conductivity Reduction by Anharmonic Rattling in Type-I Clathrates

Since the early days of studies on thermoelectric clathrates it has been evidenced that the lowering of thermal conductivity in these compounds is related to the guest ions rattling characteristics. Here we report on the relation between the guest vibration and lattice thermal conductivity κ L in single crystalline samples of type-I clathrates Ba 8 In 16 Ge 30- x M x (M = Si, Sn) with n-type carriers. On going from M x = Si 1.6 to M x = Sn 2.2 , the lattice parameter increases by 1% from 11.20 to 11.30 A. This is accompanied by a decrease from 39 to 35 K in the local vibration energy of the Ba guest ions in the tetrakaidecahedron cages, and a strong suppression of κ L , which shows glasslike temperature dependence for M x = Sn 2.2 . We further demonstrate that the lowering of κ L at 150 K shows universality behavior and scales very well with the guest free space for vibration in all type-I clathrates, largely independent of the guest or framework ions.

Phonon Dynamics of Type-I Clathrate Sr8Ga16Ge30 Studied by Inelastic Neutron Scattering

The phonon dynamics of type-I clathrates Sr 8 Ga 16 Ge 30 have been studied at room temperature by inelastic neutron scattering. Optical phonons associated with large vibrations of Sr atoms in large tetrakaidecahedral cages, (Ga,Ge) 24 , are observed at a relatively low energy of E ∼4 meV. Analysis based on a Born–von Karman force model shows that the longitudinal force constants between the Sr atoms and the oversized cages has a small value of about 0.008 mdyn/A. The results indicate that the Sr atoms are very loosely bound to the surrounding oversized cages.

Specific-Heat Measurement above 3 GPa using a Bridgman Anvil Cell

We developed an AC calorimeter using a Bridgman anvil cell. As for the thermometer and the heater, two chip resistors were mounted on the felly of the Cu–Be gasket. Thereby, the thermometer is free from pressure. In order to isolate thermally the gasket from the anvil, diamond powder (grain size <1 µm) was laid on the anvil top. The jumps in the specific heat of superconducting transitions for indium (4 mg) and tin (3 mg) samples were observed under pressures up to 3.5 GPa as clearly as at ambient pressure.

Optical Conductivity of CeNiSn, CeRhSb, and CeRhAs

The optical reflectivity spectra of the Kondo semiconductors CeNiSn, CeRhSb, and CeRhAs have been measured in a wide photon energy range to obtain the optical conductivity spectra σ(ω). For CeNiSn and CeRhSb, two pronounced peaks in the mid-infrared (IR) region and small pseudogaps in the far-IR region were observed at low temperatures ( T ). The two peaks are attributed to optical transitions related with the two bands arising from the spin–orbit splitting of Ce 4 f . For CeRhAs, σ(ω) exhibits a gap which develops gradually with cooling. The observed (pseudo)gap widths and T dependent variations are largest for CeRhAs and smallest for CeNiSn, well corresponding to their Kondo temperatures. For CeNiSn and CeRhSb, the T dependence of σ(ω) in the far-IR suggests the formation of a narrow Drude peak caused by a c – f mixing. The results may indicate that the pseudogap is opened in a heavy quasiparticle band.

Structural, Magnetic, and Superconducting Properties of Caged Compounds ROs2Zn20 (R = La, Ce, Pr, and Nd)

The electrical resistivity, magnetization, and specific heat of the caged compounds ROs2Zn20 (R = La, Ce, Pr, and Nd) have been measured to study their structural, magnetic, and superconducting properties. These measurements indicate that the compounds undergo structural transitions at Ts = 151, 109, 87, and 62 K, respectively. The decrease in Ts along with the lanthanoid contraction suggests that the high-temperature phase is more stable for a smaller lattice volume. Analysis of the lattice specific heat of LaOs2Zn20 reveals that the Zn atom at the 16c site encapsulated in the R2Zn12 cage vibrates at a low energy of 3 meV. For CeOs2Zn20, the small magnetic susceptibility with a broad maximum indicates the valence-fluctuating state of the Ce ions. In PrOs2Zn20, the crystalline electric field ground state of the 4f2 state of the Pr3+ ion remains in a non-Kramers doublet at T > Ts, which is lifted by symmetry lowering of the Pr site at T < Ts. Thereby, the quadrupolar degrees of freedom are quenched, avoidi...

Elastic Properties of Cage Compound CeOs2Al10

To investigate an origin of the novel phase transition at T 0 = 28.5 K in the cage compound CeOs 2 Al 10 , we measured temperature dependences of elastic moduli. We found a sudden elastic hardening below T 0 in all moduli and an abrupt elastic softening at T 0 in C 11 and C 22 . These behaviors in CeOs 2 Al 10 are almost similar to the elastic behaviors in CeRu 2 Al 10 , suggesting similarity of the phase transition among them. The magnitude of hardening in C L011 is the largest in the longitudinal moduli. Our results suggest that the phase transition is not simple magnetic one and the strain strongly couples to an order parameter. We estimated the strain dependence of T 0 along the a - and b -axes from the Ehrenfest relation, and that along the b -axis is the largest.

Raman Scattering Study of Filled Skutterudite Compounds

Raman scattering of filled skutterudite compounds RT 4 X 12 (R = La, Ce, Pr, Nd, Sm and Yb, T = Fe, Ru and Os, X = P and Sb) have been measured. All first order Raman active phonons are observed and are assigned as the pnicogen vibrations. In addition, at the low energy region, the 2nd order phonons due to the local vibrations of the rare-earth ions are also observed for RRu 4 P 12 and ROs 4 Sb 12 . The appearance of the 2nd order phonon in Raman spectrum is caused by a large amplitude vibration of the rare-earth ion in large cage space. This rare-earth vibration and pnicogens one, which changes the R–X bond length, show the significant resonance scattering depending on the incident energy except for the semiconductive CeFe 4 P 12 . This result suggests the existance of the interaction between the R–X stretching-like vibration and conduction electron.

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.