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-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.
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O-NMR studyTokunaga, Yo; Homma, Yoshiya*; Kambe, Shinsaku; Aoki, Dai*; Sakai, Hironori; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu; Walstedt, R. E.; Yasuoka, Hiroshi
Physica B; Condensed Matter, 359-361, p.1096 - 1098, 2005/04
Times Cited Count：7 Percentile：33.79(Physics, Condensed Matter)In order to elucidate the nature of the exotic ordered phase of NpO below 
= 26 K, we have initiated the first O-NMR measurements. From the temperature dependence of the O-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below . This finding verifies the lowering of symmetry in the ordered state of NpO.
