Experimental Electronic Heat Capacities ofα- andδ-Plutonium: Heavy-Fermion Physics in an Element

Abstract

We have measured the heat capacities of $\delta \mathrm{\text{−}}{\mathrm{P}\mathrm{u}}_{0.95}{\mathrm{A}\mathrm{l}}_{0.05}$ and $\alpha \mathrm{\text{−}}\mathrm{P}\mathrm{u}$ 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 $\delta \mathrm{\text{−}}{\mathrm{P}\mathrm{u}}_{0.95}{\mathrm{A}\mathrm{l}}_{0.05}$; we find $\gamma =64\pm 3\mathrm{m}\mathrm{J}{\mathrm{K}}^{-2}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$ as $T\to 0$. This is larger than that of any element and large enough for $\delta \mathrm{\text{−}}{\mathrm{P}\mathrm{u}}_{0.95}{\mathrm{A}\mathrm{l}}_{0.05}$ to be classed as a heavy-fermion system. By contrast, $\gamma =17\pm 1\mathrm{m}\mathrm{J}{\mathrm{K}}^{-2}{\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$ in $\alpha \mathrm{\text{−}}\mathrm{P}\mathrm{u}$. Two distinct anomalies are seen in the electronic contribution to the heat capacity of $\delta \mathrm{\text{−}}{\mathrm{P}\mathrm{u}}_{0.95}{\mathrm{A}\mathrm{l}}_{0.05}$, 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 $\delta \mathrm{\text{−}}{\mathrm{P}\mathrm{u}}_{0.95}{\mathrm{A}\mathrm{l}}_{0.05}$ may be caused by proximity to a quantum-critical point.