The rare-earth filled skutterudites are attractive systems because of their various strongly-correlated electron phenomena occurring in the same crystal structure (bcc, space group Th). In this seminar, I will present, as much as possible, those of Pr-filled skutterudites that have been studied by neutron and X-ray scattering. PrRu4P12 exhibits a metal-insulator (M-I) transition at TM-I = 63 K accompanied by a structural superlattice formation. Inelastic neutron scattering experiments elucidated two kinds of Pr ions which have different crystal-field levels in the low-temperature superlattice phase. It should be noted that the spectrum shows a strong evolution as a function of temperature. The excitation peaks shift by 3 4 meV and broaden considerably together with the level switch with approaching TM-I. The 4f-electron state in PrRu4P12 changes between a well-localized state with less amount of carriers at lower temperature and a hybridized state mediated by the p-f mixing near TM-I. PrFe4P12 shows a novel heavy-electron behavior and a non-magnetic phase transition at TA = 6.5 K. Neutron diffraction study with polarized and unpolarized beams revealed a magnetic-field-induced antiferromagnetic structure with a structural superlattice below TA. The data can be interpreted by an antiferro-quadrupolar (AFQ) ordering with the dominant ordered quadrupole O20 under magnetic field along the [001] and [110] axes. By inelastic neutron scattering, we observed sharp magnetic excitations expected to be associated with the AFQ ordering. On the other hand, the response above TA is quasielastic. This result indicates a strong hybridization between 4f electrons and carriers above TA which is consistent with the heavy-electron behavior. We have found that the novel heavy-electron superconductor PrOs4Sb12 (TC = 1.85 K) shows a characteristic evolution of magnetic excitation spectra originated from the crystal-field excitation from G1 to G4(2) near the transition temperature and a Oyz-type quadrupolar ordering under magnetic fields suppressing the superconductivity. The Pr-filled skutterudites exhibit various low-temperature behaviors, which are caused by the strong interplay between carriers and 4f electrons having not only magnetic but also multipolar degrees of freedom.
Department of Physics, Tohoku University, Sendai, Japan