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.