Takeda Naoya

Electric Field Gradient Fluctuations in Filled Skutterudite ReRu4Sb12 (Re = La, Ce, and Pr) Probed by Sb-NQR

We have carried out 121,123 Sb nuclear quadrupole resonance (NQR) measurements on Re Ru 4 Sb 12 ( Re = La, Ce and Pr) to investigate an anomalous behavior of the nuclear spin–echo relaxation rate 1/ T 2 observed in Re Os 4 Sb 12 . The full width at half maximum of the NQR spectrum shows a step-like increase below 130 K with decreasing temperature. Temperature dependence of 1/ T 2 also shows a divergence at ∼115 K whereas the nuclear spin–lattice relaxation rate 1/ T 1 does not show any anomaly. These results are caused by a distribution of the electric field gradient (EFG) and its fluctuations, which might arise from a rattling motion of the rare earth ion centered in the cage which consists of twelve Sb and/or a small deformation of the cage. The EFG fluctuations observed in Re Os 4 Sb 12 and Re Ru 4 Sb 12 at T ∼120 K originates from an unique crystal structure of the filled skutterudite with oversized Sb-cage.

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.

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.

Low-Temperature Magnetic Properties of Ho2Ni12P7

We present the magnetic susceptibility and low-temperature specific-heat of Ho 2 Ni 12 P 7 . The magnetic susceptibility of Ho 2 Ni 12 P 7 follows a Curie–Weiss law down to 10 K. Ho 2 Ni 12 P 7 orders antiferromagnetically below Neel temperature T N = 10 K. The magnetization exhibits a metamagnetic behavior below T N . The specific-heat shows a clear peak due to the transition. The crystalline-field ground state of Ho 2 Ni 12 P 7 is a doublet.

Metamagnetism of Ferromagnetic Kondo Compound SmFe4P12

SmFe 4 P 12 is a novel ferromagnetic heavy fermion compound with a Curie temperature T c of 1.6 K. The electronic specific heat coefficient is 370 mJ/mol·K 2 . The magnetic susceptibility of La 1- x Sm x Fe 4 P 12 (0.6 ≤ x ≤0.85) with a paramagnetic ground state shows a broad maximum around 15 K. This phenomenon reminds us the case of CeRu 2 Si 2 which is in the boundary of itinerant and localized 4f-state. We measured the magnetization up to 42 T and found metamagnetic crossover at 22 T. The metamagnetism disappears with increasing temperature. Possible mechanism of the metamagnetism is discussed.

NQR Study of Filled Skutterudite CeT4Sb12 (T = Ru and Os)

Sb nuclear quadrupole resonance (NQR) experiment has been performed on filled skutterudite compounds CeOs 4 Sb 12 and CeRu 4 Sb 12 in order to investigate an electronic state from a microscopic point of view. Five NQR transitions originate from 121 Sb and 123 Sb were observed and the nuclear quadrupole frequency and asymmetry parameter were estimated. 1/ T 1 T s shows exponential-like decrease with decreasing temperature above 100 K, indicating a presence of c – f hybridization gap for both compounds. 1/ T 1 T s start to increase and show characteristic temperature dependence 1/ T 1 T ∝ T - n ( n =1/2 and 1) for CeOs 4 Sb 12 and CeRu 4 Sb 12 below ∼30 K, indicating a development of the spin fluctuation. The non-Fermi-liquid behavior in CeRu 4 Sb 12 is suggested to be caused by a magnetic instability.

Seebeck Coefficient of Concentrated Kondo Compound CePtSi2 under Hydrostatic Pressure

We measured the Seebeck coefficient and resistivity of the concentrated Kondo compound CePtSi 2 under hydrostatic pressure. At ambient pressure, S gradually increases with decreasing temperature and shows a maximum value S max = 28 µV/K around 86 K, followed by a minimum around 4.5 K, as is often observed in antiferromagnetic Ce-based heavy fermion compounds. With increasing pressure, S at high temperature increases, whereas S below 20 K does not show a significant change.

Heavy Fermion Behaviors of Tm2Rh12P7

We present the magnetic susceptibility, the electrical resistivity and the low-temperature specific heat of Tm 2 Rh 12 P 7 . This compound crystallizes in the hexagonal Ho 2 Rh 12 As 7 -type structure. Rare-earth ions in this structure occupy voids of binary compound Rh 12 As 7 . This situation has a great resemblance to the relation between filled skutterudites and unfilled skutterudites. The magnetic susceptibility of Tm 2 Rh 12 P 7 follows a Curie–Weiss law down to 2 K, but there is a very small kink at 3.1 K. The specific heat shows a peak at 3.1 K, indicating a phase transition. C/T extrapolating to 0 K has a very large value of about 0.4 J/mol-Tm·K 2 . his result suggests that Tm 2 Rh 12 P 7 is the first Tm-based heavy fermion compound.

Investigation of SmFe4P12 at Pressures to 8 GPa

Electrical resistivity measurements of a ferromagnetic Kondo compound SmFe 4 P 12 have been carried out under pressures up to ∼8 GPa and at temperatures down to ∼0.3 K. With increasing pressure, shoulder-like behavior in electrical resistivity below 100 K attributed to the Kondo coherent scattering was strongly suppressed and was shifted to higher temperature region, which was often observed in Ce-based Kondo lattice compounds under applied pressure. Meanwhile, anomalous drops in resistivity ascribed to the ferromagnetic ordering have been observed at all investigated pressures up to 7.8 GPa and the values of transition temperature were hardly changed under applied pressures up to 7.8 GPa.