Heavy Fermion State in the Pr-based Skutterudites

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1. Pr-based heavy fermion compound PrFe_4P_<12>(1) By neutron scattering, muSR, nuclear specific heat measurements, the anomalous ordered state appearing below 6.5 K is found to be nonmagnetic. Considering the crystalline electric field (CEF) effect, it is highly probable to be an antiferro quadrupole ordered state. Using high quality single crystalline samples, field-temperature phase diagram has been determined. By applying the resonance level model, the energy scale of the heavy fermion state (or Kondo temperature) has been determined.(2) It is found that the substitution of La ions to Pr sites suppress the ordered state and makes the heavy fermion state restored even in zero field, suggesting that the quadruple ordering and the heavy fermion state are competing each other.2. Pr based heavy fermion superconductor PrOs_4Sb_<12>(1) Double jump structure at the superconducting phase transition has been confirmed and the field-vs-temperature phase diagram has been constructed by specific heat measurements. Using muSR technique, we found an internal magnetic field appearing in the superconducting state, which is the undoubted evidence for the broken time-reversal symmetry in the superconductivity; the first known instance of its kind in metallic materials.(2) By specific heat measurements, we discovered a field-induced ordered phase and confirmed that it is an antiferro quadrupole ordered state by neutron scattering. Based on this result, we proposed the most appropriate CEF level scheme.