title: NMR evidence for higher-order multipole order parameters in NpO2.
abstract: We report a microscopic investigation of multipolar order parameters in the ordered state of NpO2 conducted via 17O NMR on a single crystal. From the angular dependence of hyperfine fields at 17O nuclei, we have obtained clear evidence for the appearance of field-induced antiferro-octupolar as well as field-induced antiferro-dipolar moments below T0 = 26 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-q type octupolar-quadrupolar ordering model for NpO2.
authors.affiliation: ASRC, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Tokunaga
authors.forename: D
authors.initials: D
authors.lastname: Aoki
authors.forename: Y
authors.initials: Y
authors.lastname: Homma
authors.forename: S
authors.initials: S
authors.lastname: Kambe
authors.forename: H
authors.initials: H
authors.lastname: Sakai
authors.forename: S
authors.initials: S
authors.lastname: Ikeda
authors.forename: T
authors.initials: T
authors.lastname: Fujimoto
authors.forename: R E
authors.initials: RE
authors.lastname: Walstedt
authors.forename: H
authors.initials: H
authors.lastname: Yasuoka
authors.forename: E
authors.initials: E
authors.lastname: Yamamoto
authors.forename: A
authors.initials: A
authors.lastname: Nakamura
authors.forename: Y
authors.initials: Y
authors.lastname: Shiokawa