Hybridization Effects and Multipole Orders in Pr Skutterudites(Yukawa International Seminar 2004 (YKIS2004) Physics of Strongly Correlated Electron Systems)

Theoretical account is given of 4f-electron dynamics and multipole orders in Pr skutterudites with particular attention to (i) mechanism of the crystalline electric field (CEF) splitting leading to a pseudo-quartet ground state; (ii) Kondo effect due to exchange interactions involving the pseudo-quartet; (iii) multipole orders in the lattice of the pseudo-quartet in magnetic field. Competition between the point-charge interaction and hybridization between 4f and conduction electrons is identified as the key for controlling the CEF splitting. It is found that one of two pseudo-spins forming the pseudo-quartet has a ferromagnetic exchange, while the other has an antiferromagnetic exchange with conduction electrons. The Kondo effect is clearly seen in the resistivity calculated by the NCA, provided the low-lying triplet above the singlet is mainly composed of the Γ 4 -type wave functions. If the weight of the Γ 5 -type is large in the triplet, the Kondo effect does not appear. This difference caused by the nature of the triplet explains the presence of the Kondo effect in PrFe 4 P 12 , and its absence in PrOs 4 Sb 12 . By taking the minimal model with antiferro-quadrupole (AFQ) and ferro-type intersite interactions for dipoles and octupoles between nearest-neighbors, the mean-field theory reproduces the overall feature of the multiple ordered phases in PrFe 4 P 12 . The AFQ order with the Γ 3 -type symmetry is found to be stable only as a mixture of O 0 2 and O 2 2 components.