Novel features in filled skutterudites containing rare-earth elements with a plural number of 4f-electrons

Wide varieties of strongly correlated electron phenomena are performed on the stage of a “filled skutterudite structure”. Especially when one of the players contains a plural number of 4f electrons, the orbital degrees of freedom play a major role as a new type of nonmagnetic and/or weak-magnetic phenomena. Several examples found in Pr- and Sm-based filled skutterudites are introduced in relation to muon spin relaxation (μSR) experiments.