Magnetic multipole order and symmetry considerations on the ground state of Np 4+ ions in NpO 2

Abstract

The ground-state symmetries of Np ions in the low temperature phase of NpO 2 are discussed with reference to the results of resonant X-ray scattering experiments suggesting the occurrence of long-range multipolar order with electric quadrupole secondary order parameter. Two models have been proposed in the literature: one based on a Γ 4 doublet (in D 3 d point group), the other on a Γ 5 ( Γ 6 ) singlet. We show, through a theoretical group analysis, that the Γ 4 doublet is not compatible with the available experimental data. Our analysis supports the hypothesis of a singlet ground state.