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

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.Comment: 4 pages, 4 figure

THE DENSITY OF LIQUID PLUTONIUM METAL

Measurements were made of the product of surface tension (ϒ) and the contact angle (θ) of liquid plutonium vs solid tantalum and of the density of liquid plutonium (p) as functions of temperature between 655 and 960°C. The following values were obtained: ϒ cos θ = 1030 - (0.967)T d/cm and p = [17.56$sub 7$ - (1.45$sub 1$ x 10$sup -3$)T] ± 0.021 g/cm$sup 3$. A single determination was also made of the liquid density of a plutonium-7.93 atomic percent iron alloy and he following value obtained: p = [16.88 - (1.36$sub 9$)T] ± 0.012 g/cm$sup 3$

Strong electron-phonon coupling in delta-phase stabilized Pu

Heat capacity measurements of the delta-phase stabilized alloy Pu-Al suggest that strong electron-phonon coupling is required to explain the moderate renormalization of the electronic density of states near the Fermi energy. We calculate the heat capacity contributions from the lattice and electronic degrees of freedom as well as from the electron-lattice coupling term and find good overall agreement between experiment and theory assuming a dimensionless electron-phonon coupling parameter of order unity, lambda ~ 0.8. This large electron-phonon coupling parameter is comparable to reported values in other superconducting metals with face-centered cubic crystal structure, for example, Pd (lambda ~ 0.7) and Pb (lambda ~ 1.5). Further, our analysis shows evidence of a sizable residual low-temperature entropy contribution, S_{res} ~ 0.4 k_B (per atom). We can fit the residual specific heat to a two-level system. Therefore, we speculate that the observed residual entropy originates from crystal-electric field effects of the Pu atoms or from self-irradiation induced defects frozen in at low temperatures.Comment: 9 pages, 11 figures, to appear in Phys. Rev.