Exotic properties in the filled skutterudite compounds and their control by uniaxial pressure

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We have kept trying to grow large and high quality single crystals of REFe_4P_<12>, RERu_4Sb_<12>, REOs_4Sb_<12> (RE=La, Ce, Pr, Nd). On the grown crystals, we have measured basic physical properties and have made cooperative researches on various important properties with many research groups.Among them, we briefly report the most important part, especially on Pr-based systems.(1) PrFe_4P_<12> : Non-magnetic CEF ground state was further confirmed by μSR measurement. A careful evaluation of the ferro- and antiferro-magnetic components in the low temperature ordered state by neutron scattering experiments at low temperatures under magnetic fields revealed the phase to be a quadrupole one. Furthermore, the ordered state was found to be unstable against a replacement of Pr by La from transport, magnetic and specific heat measurements.(2) PrOs_4Sb_<12> : From de Haas-van Alphen (dHvA), its Fermi surface was found to be close to that of the reference compounds LaOs_4Sb_<12>, reflecting the … More localized nature of f-electrons, while we confirmed highly enhanced effective mass expected from c-f hybridization. In a NMR experiment, neither coherence peak nor node structure in superconducting gap has been observed. On the contrary, from the angular dependence of thermal conductivity in magnetic fields, anisotropic gap with point nodes. In addition, there found two different phase with different symmetries in the superconducting phase. From systematic measurements and analyses of specific heat and magnetization, Γ_1 was found to be the most probable CEF-ground state.(3) PrRu_4P_<12> : It is the 1^<st> Pr-based filled skutterudite exhibiting interesting feature (non-magnetic metal-insulator transition) and a nesting model (NM) was proposed to be an origin. However, nothing has been clarified yet. We have succeeded in detecting dHvA signal, and determined the FS. We have also performed the first successful transport and specific heat experiment, which suggests only the NM cannot explain the M-I transition. Less