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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; Fujimoto, Tatsuya; Walstedt, R. E.; Yasuoka, Hiroshi

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.



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