Novel electronic states realized in the filled skutterudites containing rare earth elements with more than one 4f-electrons

In the filled skutterudite compounds (RT₄X₁₂: R rare earth, T=Fe, Ru, Os, and X=pnictogen-ions), various attractive features have been found by replacing the constituent elements, e.g., metal-insulator transition in PrRu₄P₁₂, competing heavy fermion and multipolar ordered states in PrFe₄P₁₂, heavy fermion superconductivity in PrOs₄Sb₁₂, etc. Most of these features are realized probably by the unique crystal structure; R is embedded in the cage made of 12X and eight T ions, so that the strong c-f hybridization becomes a common feature irrespective of the constituent elements. The strong c-f hybridization in skutterudites enables the strongly correlated electron behaviors even in the systems containing rare-earth elements with more than one 4f-electron. The effect of X-replacement appears most apparently as a change in c-f hybridization, as is naturally inferred from the large difference in lattice constant. In addition to the c-f hybridization effect, it was recognized that f-electron's multipolar degrees of freedom and relatively small crystalline electric field splitting are important factors to understand the aforementioned novel phenomena.