Strong electron-phonon coupling in δ -phase stabilized Pu

Heat capacity measurements of the $\ensuremath{\delta}$-phase stabilized alloy ${\mathrm{Pu}}_{0.95}{\mathrm{Al}}_{0.05}$ 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, $\ensuremath{\lambda}\ensuremath{\sim}0.8$. This large electron-phonon coupling parameter is comparable to reported values in superconducting metals with face-centered cubic crystal structure, for example, Pd $(\ensuremath{\lambda}\ensuremath{\approx}0.7)$ and Pb $(\ensuremath{\lambda}\ensuremath{\approx}1.5)$. Further, our analysis shows evidence of a sizable residual low-temperature entropy contribution, ${S}_{\mathit{res}}\ensuremath{\approx}0.4{k}_{B}$ (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.