Strong electron-phonon coupling in delta-phase stabilized Pu

TL;DR

This study investigates the electron-phonon coupling in delta-phase stabilized plutonium, finding it to be strong and comparable to known superconducting metals, with additional evidence of residual entropy possibly due to crystal-electric field effects or defects.

Contribution

The paper provides the first detailed analysis of electron-phonon coupling strength in delta-phase stabilized Pu, combining experimental heat capacity data with theoretical calculations.

Findings

Electron-phonon coupling parameter lambda ~ 0.8

Good agreement between experiment and theory for heat capacity

Evidence of residual low-temperature entropy ~ 0.4 k_B per atom

Abstract

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.