Multipole ordering in -electron systems on the basis of a - coupling scheme

Kubo, Katsunori ; Hotta, Takashi

We investigate microscopic aspects of multipole ordering in -electron systems with emphasis on the effect of lattice structure. For the purpose, we construct -electron models on three kinds of lattices, simple cubic (sc), bcc, and fcc, in a tight-binding approximation on the basis of a - coupling scheme. Then, an effective model is derived in the strong-coupling limit for each lattice with the use of second-order perturbation theory with respect to . By applying mean-field theory to such effective models, we find multipole ordered states depending on the lattice structure. For the sc lattice, a antiferro-quadrupole transition occurs at a finite temperature and at further lowering temperature, we find another transition to a ferromagnetic state. For the bcc lattice, a antiferro-octupole ordering occurs first, and then, a ferromagnetic phase transition follows it. Finally, for the fcc lattice, we find a single phase transition to the longitudinal triple- octupole ordering.