Triple-q→Octupolar Ordering inNpO2

Abstract

We report the results of resonant x-ray scattering experiments performed at the Np $M_{4,5}$ edges in $\mathrm{N}\mathrm{p}{\mathrm{O}}_{\mathrm{2}}$. Below $T_0=25\mathrm{K}$, 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-$\overrightarrow{q}$ longitudinal antiferroquadrupolar structure. Electric-quadrupole order in $\mathrm{N}\mathrm{p}{\mathrm{O}}_{\mathrm{2}}$ could be driven by the ordering at $T_0$ 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.