Muon spin rotation measurements of the superfluid density in fresh and aged superconducting PuCoGa5

We have measured the temperature dependence and magnitude of the superfluid density rho(s)(T) via the magnetic field penetration depth lambda(T) in PuCoGa5 (nominal critical temperature T-c0=18.5 K) 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 (T-c=18.25 K) and 400 days (T-c=15.0 K) of aging at room temperature. Penetration depths lambda(0)=265(5) and >= 498(10) nm are derived for the fresh and aged samples, respectively. The temperature dependence of the superfluid density rho(s)(T)/rho(s)(0)=lambda(0)(2)/lambda(T)(2) is well described in both materials by a model using d-wave gap symmetry. Within the context of a strong-coupling dirty d-wave model a zero-temperature gap value Delta(0)=3.0k(B)T(c0) is obtained in the fresh sample for a scattering rate Gamma=0.005 pi k(B)T(c0), which is consistent with Abrikosov-Gor'kov (AG) pair-breaking theory. This Delta(0) should be compared to the weak-coupling value for a clean d-wave gap Delta(0)=2.14k(B)T(c0). In the aged sample the same model yields Delta(0)=2.4k(B)T(c0) for Gamma=0.010 pi k(B)T(c0). This value of Gamma is much less than required by the AG pair-breaking formalism. Furthermore, the aged rho(s)(0) is reduced by at least 70% compared to the fresh sample, which is also incompatible with Delta T-c/T-c0 similar to 20%, according to AG theory. We conclude that the data in aged PuCoGa5 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. We show that a model calculation consistent with this assumption fits rho(s)(T) in the aged sample rather well.