Magnetic multipole order and symmetry considerations on the ground state of Np4+ ions in NpO2

International audience; The ground-state symmetries of Np ions in the low temperature phase of NpO2 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 View the MathML source doublet (in D3d point group), the other on a View the MathML source (View the MathML source) singlet. We show, through a theoretical group analysis, that the View the MathML source doublet is not compatible with the available experimental data. Our analysis supports the hypothesis of a singlet ground state.