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 ρ_s(T) via the magnetic field penetration depth λ(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 λ(0)=265(5) and ⩾498(10) nm are derived for the fresh and aged samples, respectively. The temperature dependence of the superfluid density ρ_s(T)∕ρ_s(0)=λ(0)²∕λ(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 Δ₀=3.0k_BT_c0 is obtained in the fresh sample for a scattering rate Γ=0.005πk_BT_c0, which is consistent with Abrikosov-Gor’kov (AG) pair-breaking theory. This Δ₀ should be compared to the weak-coupling value for a clean d-wave gap Δ₀=2.14k_BT_c0. In the aged sample the same model yields Δ₀=2.4k_BT_c0 for Γ=0.010πk_BT_c0. This value of Γ is much less than required by the AG pair-breaking formalism. Furthermore, the aged ρ_s(0) is reduced by at least 70% compared to the fresh sample, which is also incompatible with ΔT_c∕T_c0∼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 ρ_s(T) in the aged sample rather well.