Muon spin rotation measurements of the superfluid density in fresh and aged superconducting PuCoGa

Oishi, Kazuki; Heffner, R. H.; Morris, G. D.*; Bauer, E. D.*; Graf, M. J.*; Zhu, J.-X.*; Morales, L. A.*; Sarrao, J. L.*; Fluss, M. J.*; MacLaughlin, D. E.*; Shu, L.*; Higemoto, Wataru  ; Ito, Takashi   

We have measured the temperature dependence and magnitude of the superfluid density via the magnetic field penetration depth in PuCoGa using the muon spin rotation technique in order to investigate the symmetry of the order parameter, and to study the effects of aging on the superconducting properties of a radioactive material. The same single crystals were measured after 25 days ( K) and 400 days ( K) of aging at room temperature. The temperature dependence of the superfluid density is well described in both materials by a model using -wave gap symmetry. Within the context of a strong-coupling, dirty -wave model, a zero-temperature gap value is obtained in the fresh sample for a scattering rate , which is consistent with Abrikosov-Gor'kov (AG) pair-breaking theory. In the aged sample the same model yields for . This value of is much less than required by the AG pair-breaking formalism. Furthermore, the aged is reduced by at least 70% compared to the fresh sample, which is also incompatible with 20%, according to AG theory. We conclude that the data in aged PuCoGa support the postulate that the scattering from radiation-induced defects is not in the limit of the AG theory of an order parameter which is spatially averaged over impurity sites, but rather in the limit of short-coherence-length superconductivity.