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-electron systemsKubo, Katsunori; Hotta, Takashi
Physica B; Condensed Matter, 378-380, p.1081 - 1082, 2006/05
Times Cited Count：4 Percentile：22.76(Physics, Condensed Matter)In order to investigate multipole ordering in -electron systems from a microscopic viewpoint, we study the so-called 
models on three kinds of lattices, simple cubic (sc), bcc, and fcc, based on a 
- coupling scheme with -electron hopping integrals through 
bonding. From the model, we derive an effective model for each lattice structure by using the second-order perturbation theory with respect to . By further applying mean-field theory to the effective model, we find a 
antiferro-quadrupole transition for the sc lattice, a 
antiferro-octupole transition for the bcc lattice, and a longitudinal triple-
octupole transition for the fcc lattice.
-electron systems on the basis of a - coupling schemeKubo, Katsunori; Hotta, Takashi
Physical Review B, 72(14), p.144401_1 - 144401_12, 2005/10
Times Cited Count：30 Percentile：72.79(Materials Science, Multidisciplinary)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.
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model for octupole ordering in NpO
Kubo, Katsunori; Hotta, Takashi
Physical Review B, 72(13), p.132411_1 - 132411_4, 2005/10
Times Cited Count：37 Percentile：77.56(Materials Science, Multidisciplinary)In order to examine the origin of octupole ordering in NpO, we propose a microscopic model constituted of neptunium 
and oxygen 
orbitals. To study multipole ordering, we derive effective multipole interactions from the - model by using the fourth-order perturbation theory in terms of - hopping integrals. Analyzing the effective model numerically, we find a tendency toward antiferro-octupole ordering.
Kubo, Katsunori; Hotta, Takashi
Physical Review B, 71(14), p.140404_1 - 140404_4, 2005/04
Times Cited Count：62 Percentile：87.28(Materials Science, Multidisciplinary)In order to examine the mysterious ordered phase of NpO from a microscopic viewpoint, we investigate an -electron model on an fcc lattice constructed based on a - coupling scheme. First, an effective model with multipole interactions is derived in the strong-coupling limit. Numerical analysis of the model clearly indicates that the interactions for 
and moments are relevant to the ground state. Then, by applying mean-field theory to the simplified model including only such interactions, we conclude that longitudinal triple- octupole order is realized in NpO through the combined effects of multipole interactions and anisotropy of the moment.
