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; Comparison with UO
from 
O-NMRTokunaga, Yo; Homma, Yoshiya*; Kambe, Shinsaku; Aoki, Dai*; Ikushima, Kenji; Sakai, Hironori; Ikeda, Shugo; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu; et al.
Journal of the Physical Society of Japan, 75(Suppl.), p.33 - 35, 2006/08
The low-temperature phase transition in NpO has attracted much attention as a promising candidate for a new class of phase transitions associated with octupolar degrees of freedom. In order to elucidate the nature of this exotic phase transition in NpO, we have initiated the first O-NMR measurements on this system using a powder sample. From the O-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below 
= 26 K. It has also been shown that the temperature and field dependence of the O NMR spectrum can be understood by considering an unconventional hyperfine interaction between the O nuclear spins and field-induced antiferromagnetic moments arising from the longitudinal triple-
antiferro-quadrupolar order. These NMR results give strong evidence for the occurrence of the longitudinal triple- multipole structure in NpO. We will also discuss our recent O-NMR results in a single crystal.
probed by NMRTokunaga, Yo; Homma, Yoshiya*; Kambe, Shinsaku; Aoki, Dai*; Sakai, Hironori; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu; Walstedt, R. E.; Yasuoka, Hiroshi
Physica B; Condensed Matter, 378-380, p.929 - 930, 2006/05
Times Cited Count：3 Percentile：17.99(Physics, Condensed Matter)We have initiated the first O-NMR measurements in the ordered state of NpO. From the O-NMR spectrum, we have confirmed the occurrence of two inequivalent oxygen sites below = 26 K. It has also been shown that the O-NMR spectrum in the ordered state can be understood by considering an unconventional hyperfine interaction between the O nuclear spins and field-induced antiferromagnetic moments arising from the longitudinal triple-
antiferro-quadrupolar order. These NMR results give strong evidence for the occurrence of the longitudinal triple- multipole structure in NpO.
Kubo, Katsunori; Hotta, Takashi
Journal of the Physical Society of Japan, 75(1), p.013702_1 - 013702_4, 2006/01
Times Cited Count：11 Percentile：56.33(Physics, Multidisciplinary)Effects of orbital degeneracy on magnetic susceptibility in paramagnetic phases are investigated within a mean-field theory. Under certain crystalline electric fields, the magnetic moment consists of two independent moments, e.g., spin and orbital moments. In such a case, the magnetic susceptibility is given by the sum of two different Curie-Weiss relations, leading to the deviation from the Curie-Weiss law. Such behavior may be observed in 
- and 
-electron systems with 
and 
ground states, respectively. As a potential application of our theory, we attempt to explain the difference in the temperature dependence of magnetic susceptibilities of UO and NpO. Then, we find that the difference in the temperature dependence can be naturally explained by considering the effects of the octupole moments in NpO.
-
model for octupole ordering in NpOKubo, 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.
multipole structure in NpOTokunaga, Yo; Homma, Yoshiya*; Kambe, Shinsaku; Aoki, Dai*; Sakai, Hironori; Yamamoto, Etsuji; Nakamura, Akio; Shiokawa, Yoshinobu; Walstedt, R. E.; Yasuoka, Hiroshi
Physical Review Letters, 94(13), p.137209_1 - 137209_4, 2005/04
Times Cited Count：78 Percentile：90.09(Physics, Multidisciplinary)In order to elucidate the nature of the exotic ordered phase of NpO below = 26 K, we have initiated the first O-NMR measurements on this system. From the O-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below . It has also been shown that the characteristic features of the hyperfine interaction at the oxygen sites are well explained by invoking a hyperfine interaction with field-induced antiferromagnetic moments which appear as a result of the triple- antiferro-quadrupolar order. The NMR findings strongly support the occurrence of the longitudinal triple- multipole structure in NpO.
