Experimental electronic heat capacities of $\alpha -$ and $\delta -$Plutonium; heavy-fermion physics in an element

Abstract

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

arXiv comment4 pages, 4 figures

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