Strongly correlated phenomena in Pr-based filled skutterudite compounds detected by neutron scattering

Recent neutron scattering studies on the Pr-based filled skutterudite compounds, which show strongly correlated phenomena, are reviewed. It was found that the 4f electron states of PrFe4P12 and PrRu4P12 are strongly hybridized with conduction electrons. It is consistent with the heavy-fermion properties observed for PrFe4P12. These states turn out to be the quadrupole ordering state (PrFe4P12) or the charge density wave state (PrRu4P12) at low temperatures. The lowest crystal field excited state of PrOs4Sb12 was found to show an excitonic behavior, which is ascribed to the quadrupolar interaction between Pr atoms. The strong temperature dependence of the excitonic excitations through the superconducting phase transition indicates a close correlation of the excitons with the superconductivity of PrOs4Sb12. The importance of the characteristic Fermi surface topology and the effects of the p-f hybridization is discussed to understand the variety of the strongly correlated phenomena in these compounds based on the experimental results.