Magnetic excitations in NpCoGa5:: Inelastic neutron scattering experiments

We report the results of inelastic neutron scattering experiments on NpCoGa5, an isostructural analog of the PuCoGa5 superconductor. Two energy scales characterize the magnetic response in the antiferromagnetic phase. One is related to a nondispersive excitation between two crystal field levels. The other at lower energies corresponds to dispersive fluctuations emanating from the magnetic zone center. The fluctuations persist in the paramagnetic phase also, although weaker in intensity. This supports the possibility that magnetic fluctuations are present in PuCoGa5, where unconventional d-wave superconductivity is achieved in the absence of magnetic order.