title: Triple-q(-->) octupolar ordering in NpO2.
abstract: We report the results of resonant x-ray scattering experiments performed at the Np M(4,5) edges in NpO2. Below T(0)=25 K, the development of long-range order of Np electric quadrupoles is revealed by the growth of superlattice Bragg peaks. The polarization and azimuthal dependence of the intensity of the resonant peaks are well reproduced assuming anisotropic tensor susceptibility scattering from a triple-q(-->) longitudinal antiferroquadrupolar structure. Electric-quadrupole order in NpO2 could be driven by the ordering at T0 of magnetic octupoles of Gamma(5) symmetry, splitting the Np ground state quartet and leading to a singlet ground state with zero dipole-magnetic moment.
authors.affiliation: Departamento de Física, Universidade de Coimbra, P-3004 516 Coimbra, Portugal.
authors.forename: J A
authors.initials: JA
authors.lastname: Paixão
authors.forename: C
authors.initials: C
authors.lastname: Detlefs
authors.forename: M J
authors.initials: MJ
authors.lastname: Longfield
authors.forename: R
authors.initials: R
authors.lastname: Caciuffo
authors.forename: P
authors.initials: P
authors.lastname: Santini
authors.forename: N
authors.initials: N
authors.lastname: Bernhoeft
authors.forename: J
authors.initials: J
authors.lastname: Rebizant
authors.forename: G H
authors.initials: GH
authors.lastname: Lander