Triple-q octupolar ordering in NpO_2

TL;DR

This study uses resonant X-ray scattering to reveal a complex triple-q octupolar and quadrupolar order in NpO_2 below 25 K, showing no lattice distortion but a significant electronic ordering phenomenon.

Contribution

It provides experimental evidence for a triple-q antiferroquadrupolar structure driven by magnetic octupoles in NpO_2, advancing understanding of multipolar ordering in actinide compounds.

Findings

Long-range quadrupolar order develops below 25 K.

No detectable crystallographic distortion accompanies the electronic order.

The order is consistent with a triple-q antiferroquadrupolar structure.

Abstract

We report the results of resonant X-ray scattering experiments performed at the Np M_4,5 edges in NpO_2. 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 electronic transition is not accompanied by any measurable crystallographic distortion, either internal or external, so the symmetry of the system remains cubic. The polarization and azimuthal dependence of the intensity of the resonant peaks is well reproduced assuming Templeton scattering from a triple-q longitudinal antiferroquadrupolar structure. Electric quadrupole order in NpO_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.