Heavy fermion behaviors in the Pr-based filled skutterudites

In filled skutterudites, which have attracted much attention as candidate materials for thermoelectric applications, rare Pr-based heavy-fermion (HF) behaviors have been revealed recently. We review recent studies focusing on PrFe4P12 and PrOs4Sb12; the former shows HF behaviors in high-fields where a quadruple ordering is suppressed, and the latter shows the first known Pr-based HF superconductivity as well as an anomalous field-induced quadruple ordering. We suggest that quadrupole degrees of freedom on the Pr-ions commonly play key roles in these exotic states.