NMR evidence for higher-order multipole order parameters in NpO

Tokunaga, Yo  ; Aoki, Dai*; Homma, Yoshiya*; Kambe, Shinsaku  ; Sakai, Hironori   ; Ikeda, Shugo; Fujimoto, Tatsuya; Walstedt, R. E.*; Yasuoka, Hiroshi; Yamamoto, Etsuji  ; Nakamura, Akio; Shiokawa, Yoshinobu*

We report a microscopic investigation of multipolar order parameters in the ordered state of NpO conducted via O NMR on a single crystal. From the angular dependence of hyperfine fields at O nuclei, we have obtained clear evidence for the appearance of field-induced antiferro-octupolar as well as field-induced antiferro-dipolar moments below K. We have also observed oscillatory spin-echo decay, which is well understood in terms of small electric field gradients created by antiferro-quadrupolar ordering. This reveals that the quadrupolar order parameter is directly observable by means of NMR. The present NMR studies provide definitive support for a proposed longitudinal triple- type octupolar-quadrupolar ordering model for NpO.