Experimental electronic heat capacities of $\hat{I}\pm -$ and $\hat{I}´-$Plutonium; heavy-fermion physics in an element

Abstract

We have measured the heat capacities of $\hat{I}´-$Pu${}_{0.95}$Al${}_{0.05}$ and $\hat{I}\pm -$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 deduction of the electronic contribution to the heat capacity of $\hat{I}´-$Pu${}_{0.95}$Al${}_{0.05}$; we find ${\hat{I}}^3=64\pm 3$ mJK${}^{-2}$mol${}^{-1}$ as $T\to 0$. This is a factor $\sim 4$ larger than that of any element, and large enough for $\hat{I}´-$Pu${}_{0.95}$Al${}_{0.05}$ to be classed as a heavy-fermion system. By contrast, ${\hat{I}}^3=17\pm 1$ mJK${}^{-2}$mol${}^{-1}$ in $\hat{I}\pm -$Pu. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $\hat{I}´-$Pu${}_{0.95}$Al${}_{0.05}$, one or both of which may be associated with the formation of the $\hat{I}{\pm }^{\prime }-$ martensitic phase. We suggest that the large ${\hat{I}}^3$-value of $\hat{I}´-$Pu${}_{0.95}$Al${}_{0.05}$ may be caused by proximity to a quantum-critical point.

4 pages, 4 figures