Specific Heat of δ -Pu Stabilized by Am

Detailed specific heat Cₚ measurements of δ-Pu stabilized by Am (8%--20%) were performed in the temperature range 4.5--300 K. The coefficient of the electronic specific heat γ, which reflects the quasiparticle density of states at the Fermi level EF, 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, γ decreases slightly with the Am content. An applied magnetic field of 9 T had no effect on Cₚ. The results strongly suggest that itinerant $5f$ states at EF are not appropriate for describing δ-Pu.