Multipolar phase transition in NpO; Comparison with UO from 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 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.