抄録
Low-lying singlet–triplet crystal-field levels are realized in Pr-based filled-skutterudite compounds. The properties of multipolar moments in such systems are studied theoretically to elucidate the microscopic origin of their anomalous ordered states. First, we introduce a proper set of multipoles to express the active degrees of freedom in the singlet–triplet subspace. Then, a group-theoretical classification of multipoles is presented to discuss the nature of mixed order parameters in zero and finite magnetic fields. Using a pseudo-spin representation, we construct a generic form of pair interactions and identify the sources of anisotropy for directions of magnetic field. The interactions and the phase diagram of PrOs4Sb12, which is a typical singlet–triplet system, are discussed in connection with the Th symmetry specific to the filled-skutterudite systems.
