Novel Kondo Behaviors Realized in the Filled Skutterudite Structure

Abstract

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. PrRu 4 Sb 12 is an ordinary BCS superconductor without f -electron-derived mass enhancement. PrRu 4 P 12 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 PrOs 4 Sb 12 and the reference compound LaOs 4 Sb 12 , suggesting the well-localized character of 4 f -electrons in PrOs 4 Sb 12 . In contrast, the topology of the FS of PrFe 4 P 12 , 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 LaFe 4 P 12 , although partial similarity is visible between the two. For both PrFe 4 P 12 and PrOs 4 Sb 12 , 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 antiferro-electronic-quadrupole (orbital) ordering of Pr-ions are realized in zero field for PrFe 4 P 12 and under magnetic fields for PrOs 4 Sb 12 , 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.