Octupolar order in NpO2: A specific heat investigation

Resonant X-ray scattering (RXS) experiments on NpO2 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 Γ2 or with Γ5 octupoles. A consequence of long-range octupole order is that the degeneracy of the Γ8 crystal field ground state is removed. Γ8 would be split into two doublets if Γ2 octupoles order, and into two singlets and one doublet if the order parameter has Γ5 symmetry. Here we present specific heat measurements between 0.7K and 300K supporting a singlet ground state in the ordered phase below 25.5K