Theoretical research on orbital orders and fluctuations

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Theoretical study was made on the following aspects.*Microscopic study of interactions for orbital orderWe explained the origin of characteristic wave numbers in the ordering pattern of rare-earth hexaborides.*Band theory taking account of the orbital orderWe developed a new calculational scheme within the so-called LDA+U method in order to derive the band structure in the presence of orbital orders. The results clarify the relashionship between the band structure and the physical properties.*Theory of orbital degrees of freedom and magnetic order(1) CeB6 : In the quadrupole ordered phase of CeB6, we have predicted that magnetic field induces staggered octupoles. Within this project natural explanation for the phenomena was provided on the basis of analyzing interaction among Ce ions. Furthermore, we pointed out the importance of pseudo-dipolar interactions in understanding the orthogonal arrangement of nearest-neighbor moments in CeB6.(2) Yb4As3 : Charge ordering of this compound leads to a characteristic magnetic property at low temperature. We derived the effective Hamiltonian for the system, which describes accurately the anisotropy including the Dzyaloshinski-Moriya interaction.(3) TmTe : This compound has the cubic symmetry and has the crystalline-electric-field ground state. Hence we examined the character of the quadrupole order and magnetic moment induced by magnetic field, in comparsion with CeB6.