Octupolar order in [formula omitted]: A specific heat investigation

Abstract

Resonant X-ray scattering (RXS) experiments on NpO 2 provide indications for a low-temperature phase where magnetic octupole order is established in absence of magnetic dipole order. The interpretation of the RXS findings has led to a controversy about the symmetry of the primary order parameter, which has been identified either with Γ 2 or with Γ 5 octupoles. A consequence of long-range octupole order is that the degeneracy of the Γ 8 crystal field ground state is removed. Γ 8 would be split into two doublets if Γ 2 octupoles order, and into two singlets and one doublet if the order parameter has Γ 5 symmetry. Here we present specific heat measurements between 0.7 K and 300 K supporting a singlet ground state in the ordered phase below 25.5 K.