Multipole phenomena and superconductivity in Pr-based filled skutterudites

Field-orientation dependent specific heat C(H, θ) and the magnetization M(H, θ) have been measured on the Pr-based filled skutterudite compounds PrOs₄Sb₁₂ and PrFe ₄P₁₂. In PrOs₄Sb₁₂, fourfold angular oscillations of C(H, θ) have been observed in the normal state above the upper critical field H_c2 as well as in the vortex state. Field variation of the normal-state oscillation in C(H, θ) strongly suggests that antiferroquadrupolar fluctuations exist in the wide region above H _c2. The oscillation exhibits a sign reversal below H_c2, whose origin is attributed to the nodal structure in which the superconducting gap takes minima (point nodes) along [100] directions. In PrFe₄P ₁₂, an exotic ordered phase (A-phase) below T_A(H = 0)=6.5 K has been examined. Both M(H, θ) within the A-phase and T_A(H, θ) show a smooth angular variation with a cubic anisotropy for H rotated in the (001) and (110) planes. The results strongly suggest that cubic symmetry is preserved in the A-phase; the order parameter is of Γ₁ ⁺ type.