Absence of superconductivity in fluorine-doped neptunium pnictide NpFeAsO.

X-ray diffraction, specific heat, magnetic susceptibility and inelastic x-ray scattering measurements on the transurarium oxypnictides NpFeAsO and NpFeAsO0.85F0.15 are presented. No superconductivity down to 2 K was observed upon fluorine doping, contrary to the structurally analogous rare-earth pnictides. No modification of the phonon density of states was observed upon doping with fluorine. We discuss our results in light of the latest experimental and theoretical studies on the role of phonons in the superconducting pnictide compounds.

Specific heat of delta-Pu stabilized by Am.

Detailed specific heat C(p) measurements of delta-Pu stabilized by Am (8%-20%) were performed in the temperature range 4.5-300 K. The coefficient of the electronic specific heat gamma, which reflects the quasiparticle density of states at the Fermi level E(F), is smaller than originally assumed and, depending on the estimate of phonon contributions, a value between 35 and 55 mJ/mol K2 can be deduced for Pu-8% Am. For higher Am concentrations, which expand the lattice, gamma decreases slightly with the Am content. An applied magnetic field of 9 T had no effect on C(p). The results strongly suggest that itinerant 5f states at E(F) are not appropriate for describing delta-Pu.