| Research Abstract |
Systematic study of electronic structures for binary skutterudite compounds (T'X_3 : T' = Co, Rh, Ir and X = P, As, Sb and NiP_3) and ternary skutterudite compounds (RT_4X_<12> : R = rare earth, T = Fe, Ru, Os and X = P, As, Sb) are performed.
The electronic band structures are calculated for T'X_3 and La-based ternary compounds by using an FLAPW method. Except for NiP_3, binary compounds are basically semiconductors. La-based ternary compounds are the metal with one hole per formula unit. The main Fermi surface is a distorted cube and the volume is almost a half of the BCC BZ, indicating that the nesting with q=(1,0,0) is likely in ternary skutterudite compounds.
PrRu_4P_<12> shows a metal-insulator transition and PrFe_4P_<12> undergoes a non-magnetic ordering. In fact, structural phase transitions are observed in these compounds. The coupling of Fe/Ru ion displacements to the 4f^2 ground state doublet results in a unique symmetry-lowering structural phase transition accompanied by anti-quadrupolar ordering. The instability of the Fermi surface should be coupled, through Fe/Ru displacements, with anti-quadrupolar ordering at the structural phase transitions.
Very recently, a new class of heavy fermion superconductivity has been reported in PrOs_4Sb_<12>. The dHvA measurements have been performed for LaOs_4Sb_<12> and PrOs_4Sb_<12>, then it has been revealed they have very similar topology. The Fermi surface of the LDA calculation for PrOs_4Sb_<12> does not explain the experimental dHvA measurement. The LAD + U calculation (f localized case) for PrOs_4Sb_<12> strongly suggest the 4f^2 electrons are localized.
The local point symmetry of the rare earth site is T_h. For about two decades, it has been believed that the same formula of crystal electric fields is applicable for all cubic point groups. We find that the new term is non-vanishing for the cubic point groups T_h and T due to the lack of Umklappung and fourfold symmetry axis of the point group O_h.
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