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

TL;DR

This study measures the heat capacities of delta and alpha plutonium, revealing heavy-fermion behavior in delta phase and suggesting proximity to a quantum-critical point.

Contribution

First detailed heat capacity measurements of delta and alpha plutonium, identifying heavy-fermion characteristics and anomalies linked to phase transformations.

Findings

Delta-Pu has a large electronic heat capacity coefficient, indicating heavy-fermion behavior.

Two anomalies in delta-Pu heat capacity suggest phase-related phenomena.

Delta-Pu's properties imply proximity to a quantum-critical point.

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 $\alpha'-$ 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.