Experimental electronic heat capacities of alpha- and delta-plutonium: heavy-fermion physics in an element

We have measured the heat capacities of delta-Pu0.95Al0.05 and alpha-Pu over the temperature range 2-303 K. The availability of data below 10 K plus an estimate of the phonon contribution to the heat capacity based on recent neutron-scattering experiments on the same sample enable us to make a reliable deduc-tion of the electronic contribution to the heat capacity of delta-Pu0.95Al0.05; we find gamma=64+/-3 mJ K(-2) mol(-1) as T-->0. This is larger than that of any element and large enough for delta-Pu0.95Al0.05 to be classed as a heavy-fermion system. By contrast, gamma=17+/-1 mJ K(-2) mol(-1) in alpha-Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of delta-Pu0.95Al0.05, one or both of which may be associated with the formation of the alpha(')-martensitic phase. We suggest that the large gamma value of delta-Pu0.95Al0.05 may be caused by proximity to a quantum-critical point.