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Journal Articles

Multipolar phase transition in NpO$$_{2}$$; Comparison with UO$$_2$$ from $$^{17}$$O-NMR

Tokunaga, 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$$_2$$ 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$$_2$$, we have initiated the first $$^{17}$$O-NMR measurements on this system using a powder sample. From the $$^{17}$$O-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below $$T_0$$= 26 K. It has also been shown that the temperature and field dependence of the $$^{17}$$O NMR spectrum can be understood by considering an unconventional hyperfine interaction between the $$^{17}$$O nuclear spins and field-induced antiferromagnetic moments arising from the longitudinal triple-$$vec{q}$$ antiferro-quadrupolar order. These NMR results give strong evidence for the occurrence of the longitudinal triple-$$vec{q}$$ multipole structure in NpO$$_2$$. We will also discuss our recent $$^{17}$$O-NMR results in a single crystal.

Journal Articles

Multipolar ordering in NpO$$_2$$ probed by NMR

Tokunaga, 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:18.63(Physics, Condensed Matter)

We have initiated the first $$^{17}$$O-NMR measurements in the ordered state of NpO$$_2$$. From the $$^{17}$$O-NMR spectrum, we have confirmed the occurrence of two inequivalent oxygen sites below $$T_0$$= 26 K. It has also been shown that the $$^{17}$$O-NMR spectrum in the ordered state can be understood by considering an unconventional hyperfine interaction between the $$^{17}$$O nuclear spins and field-induced antiferromagnetic moments arising from the longitudinal triple-$$bf{q}$$ antiferro-quadrupolar order. These NMR results give strong evidence for the occurrence of the longitudinal triple-$$bf{q}$$ multipole structure in NpO$$_2$$.

Journal Articles

Magnetic susceptibility of multiorbital systems

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:57.37(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 $$d$$- and $$f$$-electron systems with $$t_{2g}$$ and $$Gamma_8$$ 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$$_2$$ and NpO$$_2$$. Then, we find that the difference in the temperature dependence can be naturally explained by considering the effects of the octupole moments in NpO$$_2$$.

Journal Articles

Analysis of $$f$$-$$p$$ model for octupole ordering in NpO$$_2$$

Kubo, Katsunori; Hotta, Takashi

Physical Review B, 72(13), p.132411_1 - 132411_4, 2005/10

 Times Cited Count:37 Percentile:78.66(Materials Science, Multidisciplinary)

In order to examine the origin of octupole ordering in NpO$$_2$$, we propose a microscopic model constituted of neptunium $$5f$$ and oxygen $$2p$$ orbitals. To study multipole ordering, we derive effective multipole interactions from the $$f$$-$$p$$ model by using the fourth-order perturbation theory in terms of $$p$$-$$f$$ hopping integrals. Analyzing the effective model numerically, we find a tendency toward $$Gamma_{5u}$$ antiferro-octupole ordering.

Journal Articles

Microscopic theory of multipole ordering in NpO$$_2$$

Kubo, Katsunori; Hotta, Takashi

Physical Review B, 71(14), p.140404_1 - 140404_4, 2005/04

 Times Cited Count:60 Percentile:87.62(Materials Science, Multidisciplinary)

In order to examine the mysterious ordered phase of NpO$$_2$$ from a microscopic viewpoint, we investigate an $$f$$-electron model on an fcc lattice constructed based on a $$j$$-$$j$$ 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 $$Gamma_{4u}$$ and $$Gamma_{5u}$$ 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-$$q$$ $$Gamma_{5u}$$ octupole order is realized in NpO$$_2$$ through the combined effects of multipole interactions and anisotropy of the $$Gamma_{5u}$$ moment.

Journal Articles

NMR evidence for triple-$$overrightarrow{q}$$ multipole structure in NpO$$_2$$

Tokunaga, 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:74 Percentile:90.1(Physics, Multidisciplinary)

In order to elucidate the nature of the exotic ordered phase of NpO$$_2$$ below $$T_0$$= 26 K, we have initiated the first $$^{17}$$O-NMR measurements on this system. From the $$^{17}$$O-NMR spectrum, the occurrence of two inequivalent oxygen sites has been confirmed below $$T_0$$. 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-$$vec{q}$$ antiferro-quadrupolar order. The NMR findings strongly support the occurrence of the longitudinal triple-$$vec{q}$$ multipole structure in NpO$$_2$$.

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