Role of self-irradiation defects on the ageing of ²³⁹ PuCoGa ₅

Abstract

Abstract. -Low temperature accumulation and annealing experiments, in conjunction with electrical resitivity and critical current density measurements, are used to study the ageing of the actinide superconductor PuCoGa5. These measurements reveal that 2-nm sized nonsuperconducting point-like regions are the main damage formed during room temperature ageing; smaller point like defects are irrelevant to transport properties. Defect sizes and densities deduced from experiment agree with Transmission Electron Microscopy observations.

Possible mechanism of superconductivity inPuCoGa5probed by self-irradiation damage

“…This long aging time was required to observe a decrease of the critical temperature T c below our experimental limit of 1.8 K. Moreover, a deviation from linearity in the time dependence of T c and of the electrical resistivity has been observed, whereas previous studies over shorter periods did not allow one to test this behavior. 8 To put this finding into context, we show that the observations are consistent to what is expected from Eliashberg model for superconductivity, [9][10][11][12] and that all the most significant experimental data existing for this compound can be reproduced assuming a phononic mechanism with reasonable values of the electron-phonon coupling and Coulomb pseudopotential. This does not exclude a possible role of spin fluctuations, but it does show that an electron-phonon coupling model involving a minimal number of free parameters can well describe the main physical behavior of PuCoGa 5 , provided that too simple approximations are avoided.…”

Unconventional superconductivity

“…This dependence on the water content, and the tendency of the water to evaporate at ambient conditions, makes this compound the material with the most sample dependence of any in this review. It displays also the extremal aging dependence of any sample discussed herein: while 239 PuCoGa5, Tc=18.5 K, due to selfirradiation damage, shows [431] a diminishing Tc (Tc ~ -0.5 K in one month), Nax(H3O)zCoO2  yH2O shows a superconductivity that changes on the scale of (two) days (Oeschler et al [664].) Within 40 days at ambient conditions, the sample is entirely non-superconducting.…”

“…(Although PuCoGa5, Tc=18.5 K, and PuRhGa5, Tc=8.7 K, have higher Tc values, their  values do not really qualify them as 'heavy' Fermion systems. Plus, the problems of substitution using the extremely radioactive Pu isotopes and the attendant Tc decay with time (Tc ~ -0.5 K/month) due to self-irradiation damage effects [431] seem insurmountable obstacles to determining the isotope effect accurately enough. )…”

Nature of the5fstates in actinide metals

“…Evidence of this is given by the fact that self-induced irradiation damage causes the superconducting transition temperature to drop ~0.2 K per month in PuCoGa 5 , slowing in rate as defects accumulate over time. Booth et al, 2007;Jutier et al, 2007a and. PuCoGa 5 and the 115 compounds will be further discussed in the section on pressure-dependent superconductivity of Am metal.…”