| Research Abstract |
We have prepared the high-quality single crystals of filled skutterudite RT_4X_<12> (R=rare earth, T=Fe,Ru,Os,X=P,As,Sb) and investigated the anomalous physical properties. Main results are summarized as below.
1.Heavy fermion superconductivity of PrOs_4Sb_<12>
2.Metal-insulator (M-I) transition of PrRu_4P_<12>
3.Unusual heavy fermion state of SmOs_4Sb_<12>
About PrOs_4Sb_<12>, we have measured the magnetoresistance for field along the three principal field directions and determined the magnetic field-temperature phases, from which we have confirmed the crystal electric field (CEF) ground state of Pr^3+ to be Γ_1 singlet. From inelastic neutron diffraction experiments, we have found the narrowing of the line widths of the magnetic excitations below the superconducting transition temperature, which is suggested the close connection between the superconductivity and the excitations. From the thermal conductivity measurements, we have found an anomalous feature of magnetic field dependence, f
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rom which we have proposed the multi band superconductivity of PrOs_4Sb_<12>.
About PrRu_4P_<12>, to clarify the origin of M-I transition, we have measured the de Haas-van Alphen (dHvA) effect of the reference compound LaRu_4P_<12> and determined the Fermi surface (FS). We have found that the FS of LaRu_4P_<12> is basically the same with that of calculated FS of PrRu_4P_<12>, that is consistent with the localized nature of 4f electrons in PrRu_4P_<12>. Taking into account that LaRu_4P_<12> does not show the M-I transition, we have pointed out the essential role of correlation between the 4f- and conduction electrons cooperating with the FS nesting in driving the M-I transition of PrRu_4P_<12>. From inelastic neutron diffraction experiments, we have found that the 4f electron state far below the M-I transition temperature is described as well-defined CEF excitations, i.e., two different Pr-sites have different CEF each other, which explains the magnetic properties.
About SmOs_4Sb_<12>, we have confirmed the bulk ferromagnetism below 3 K from NQR measurement, and found that this compounds is situated at the vicinity of ferromagnetic critical point in the pressure-temperature phase diagram from the resistivity measurements under pressure.
For other filled skutterudites, such as SmRu_4P_<12> (M-I transition), CeOs_4Sb_<12> (Kondo semiconductor) etc., various experiments, such as electronic transport, dHvA effect, thermal and magnetic experiments, neutron diffraction, μSR,NMR. NQR, photoemission, optical conductivity, elastic, Mossbauer and Ramman scattering experiments are in progress to clarify the origin of the anomalous physical properties. Less
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