Sato Hideyuki

On the Symmetry of Low-Field Ordered Phase of PrFe4P12: 31P NMR

We have performed 31 P nuclear magnetic resonance (NMR) experiments on the filled skutterudite compound PrFe 4 P 12 to investigate the exotic ordered phase at low temperatures and low magnetic fields. Analysis of the NMR line splitting in the ordered phase indicates that totally symmetric (Γ 1 -type) staggered magnetic multipoles are induced by magnetic fields. This is incompatible with any type of quadrupole order at zero field. We conclude that the ordered phase has broken translational symmetry with the wave vector Q =(1,0,0) but the T h point symmetry at the Pr sites is not broken by the non-magnetic Γ 1 -type order parameter.We have performed 31P nuclear magnetic resonance (NMR) experiments on the filled skutterudite compound PrFe4P12 to investigate the exotic ordered phase at low temperatures and low magnetic fields. Analysis of the NMR line splitting in the ordered phase indicates that totally symmetric (\Gamma_1-type) staggered magnetic multipoles are induced by magnetic fields. This is incompatible with any type of quadrupole order at zero field. We conclude that the ordered phase has broken translational symmetry with the wave vector Q=(1,0,0) but the Th point symmetry at the Pr sites is not broken by the non-magnetic \Gamma_1-type order parameter.

Novel Features Realized in the Filled Skutterudite Structure

Full varieties of strongly correlated electron phenomena have been found in the filled skutterudite systems, by changing the constituent elements. The unique crystal structure leads to novel features even in the systems containing rare earth elements with plural number of 4 f -electrons. According to the intensive competitions and cooperation among experimental and theoretical research works, remarkable progress has been made in sorting out several basic origins that bring about the variety of this system, such as the strong c – f hybridization, the small CEF level splitting, the orbital degree of freedom (multipoles), and the positional degree of freedom within a pnictogen cage. However, there still remain many attractive questions to be answered.

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.

Single Crystal Growth and Electrical Properties of CeRh2 and CeIr2

We have grown single crystals of CeRh 2 and CeIr 2 by a tri-arc Czochralski pulling method. Electrical properties including the de Haas-van Alphen effect are presented.

Anisotropic Magnetic Properties of CeCu2

We have measured the electrical resistivity, Hall coefficient, thermoelectric power, magnetoresistance, magnetic susceptibility and magnetization of the antiferromagnetic Kondo-lattice compound CeCu 2 using a high-quality single crystal. Anisotropy reflected in the above quantities is explained by the orthorhombic crystalline electric field. We discuss the metamagnetic behavior and the negative magnetoresistance which scales with a square of the magnetization.

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.

Optical Conductivity and Electronic Structures in Ce-Filled Skutterudites

Optical conductivity [σ(ω)] spectra have been measured for eight Ce-filled skutterudite compounds Ce T 4 X 12 ( T = Fe, Ru, Os; X = P, As, Sb) to systematically study their microscopic electronic structures around Fermi level. The σ(ω) spectra show large differences among X = P, As and Sb, while they show smaller differences among T = Fe, Ru and Os for a given X element. This indicates that the choice of X has larger effects on the electronic structures of Ce T 4 X 12 than that of T . The spectral shape and temperature dependence of σ(ω) suggest that the electronic structures of Ce T 4 P 12 are similar to that of a band semiconductor and those of Ce T 4 Sb 12 to that of a heavy fermion metal or a Kondo semiconductor. In the case of Ce T 4 As 12 , the σ(ω) spectra suggest intermediate characteristics between Ce T 4 P 12 and Ce T 4 Sb 12 , namely they are similar to those shown by a semiconductor with a very narrow energy gap.

Effects of Filling Fraction on Magnetic Properties of Filled Skutterudite RFe4Sb12 (R = La, Ce) Synthesized under High Pressure

We report the results of the 121,123 Sb-nuclear quadrupole resonance (NQR) measurements on the filled skutterudite R Fe 4 Sb 12 ( R = La, Ce) synthesized under high pressure (HP), and compare them with those for the sample synthesized under ambient pressure (AP) in order to understand the effects of the R -site filling fraction on magnetic properties. The NQR spectra for the HP samples are much sharper than those for the AP samples, suggesting that the R -site filling fraction is high close to unity and the electronic states around Sb nuclei become uniform. In LaFe 4 Sb 12 -HP, the nuclear spin–lattice relaxation rate divided by temperature, 1/ T 1 T , exhibits the Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with the Weiss temperature θ=12 K above 30 K, indicating the development of ferromagnetic fluctuations. The magnitude of θ for the HP sample is smaller than that for the AP sample, which suggests that the system becomes close to the vicinity of ferromagnetic instability as the La-site fil...

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.