Muon spin rotation measurements of the superfluid density in fresh and aged superconducting<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mi mathvariant="normal">Pu</mml:mi><mml:mi mathvariant="normal">Co</mml:mi><mml:msub><mml:mi mathvariant="normal">Ga</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:mrow></mml:math>

We have measured the temperature dependence and magnitude of the superfluid density ${\ensuremath{\rho}}_{s}(T)$ via the magnetic field penetration depth $\ensuremath{\lambda}(T)$ in $\mathrm{Pu}\mathrm{Co}{\mathrm{Ga}}_{5}$ (nominal critical temperature ${T}_{c0}=18.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) 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 $25\phantom{\rule{0.3em}{0ex}}\text{days}$ $({T}_{c}=18.25\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and $400\phantom{\rule{0.3em}{0ex}}\text{days}$ $({T}_{c}=15.0\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ of aging at room temperature. Penetration depths $\ensuremath{\lambda}(0)=265(5)$ and $\ensuremath{\geqslant}498(10)\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ are derived for the fresh and aged samples, respectively. The temperature dependence of the superfluid density ${\ensuremath{\rho}}_{s}(T)∕{\ensuremath{\rho}}_{s}(0)=\ensuremath{\lambda}{(0)}^{2}∕\ensuremath{\lambda}{(T)}^{2}$ 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 ${\ensuremath{\Delta}}_{0}=3.0{k}_{B}{T}_{c0}$ is obtained in the fresh sample for a scattering rate $\ensuremath{\Gamma}=0.005\ensuremath{\pi}{k}_{B}{T}_{c0}$, which is consistent with Abrikosov-Gor'kov (AG) pair-breaking theory. This ${\ensuremath{\Delta}}_{0}$ should be compared to the weak-coupling value for a clean $d$-wave gap ${\ensuremath{\Delta}}_{0}=2.14{k}_{B}{T}_{c0}$. In the aged sample the same model yields ${\ensuremath{\Delta}}_{0}=2.4{k}_{B}{T}_{c0}$ for $\ensuremath{\Gamma}=0.010\ensuremath{\pi}{k}_{B}{T}_{c0}$. This value of $\ensuremath{\Gamma}$ is much less than required by the AG pair-breaking formalism. Furthermore, the aged ${\ensuremath{\rho}}_{s}(0)$ is reduced by at least 70% compared to the fresh sample, which is also incompatible with $\ensuremath{\Delta}{T}_{c}∕{T}_{c0}\ensuremath{\sim}20%$, according to AG theory. We conclude that the data in aged $\mathrm{Pu}\mathrm{Co}{\mathrm{Ga}}_{5}$ 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 ${\ensuremath{\rho}}_{s}(T)$ in the aged sample rather well.

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