Novel Kondo behaviors realized in the filled skutterudite structure

Novel Kondo-like behaviors realized in the Pr-based filled skutterudites are reviewed based on the electronic transport properties, specific heat and de Haas-van Alphen (dHvA) experiments, along with the band structure calculations. As references, the experiments on the related La- and Ce-filled skutterudites are also studied and discussed. PrRu4Sb12 is an ordinary BCS superconductor without P electron-derived mass enhancement. PrRu4P12 exhibits a metal-semiconductor transition at 62 K and an anomaly with a small characteristic energy at low temperatures. The Fermi surface (FS) topology is very similar between the first known heavy fermion superconductor PrOs4Sb12 and the reference compound LaOs4Sb12, suggesting the well-localized character of 4f-electrons in PrOs4Sb12, In contrast, the topology of the FS of PrFe4P12, which exhibits apparent Kondo behaviors at high temperatures and the field-induced heavy fermion state at low temperatures, is as a whole different from that of LaFe4P12, although partial similarity is visible between the two. For both PrFe4P12 and PrOs4Sb12, de Haas-van Alphen experiments revealed strongly enhanced cyclotron effective masses, in reasonable agreement with the large specific heat coefficients. The ground state is settled to be non-magnetic, and anomalous antiferroelectronic-quadrupole (orbital) ordering of Pr-ions are realized in zero field for PrFe4P12 and under magnetic fields for PrOs4Sb12, suggesting that the quadrupole degrees of freedom play key roles in the heavy-fermion and the exotic superconducting behaviors. The novel features realized in the Pr-filled skutterudites are discussed in relation with the unique crystal structures.