$\mu $SR measurement of the superconducting penetration depth in aged PuCoGa$_{5}$

The superconductor PuCoGa$_{5}$ possesses a relatively high critical temperature (T$_{c}$ = 18.5 K), a moderate mass enhancement (Sommerfeld coefficient $\cong $ 80 mJ/mol-K$^{2})$ and exhibits behavior consistent with spin-fluctuation-induced d-wave superconductivity [1]. It is also radioactive, with its dominant radioisotope $^{239}$Pu having a half-life of 2.4 x 10$^{4}$ yrs. We report $\mu $SR measurements of the temperature and field dependence of the magnetic field penetration depth $\lambda $(T,H) in a 400 day-old sample of PuCoGa$_{5}$ (T$_{c}$ = 15 K), and compare these results to data obtained on the same sample approximately one year earlier. In 600 Oe applied field both measurements show a linear low-temperature behavior for $\lambda $(T) - $\lambda $(0) = bT for T/T$_{c} \quad <$ 0.5 with the same coefficient b. The magnitude of $\lambda $(0) in the aged sample has increased by a factor of about 1.8. The results are discussed in terms of the effects of radiation damage on the postulated d-wave superconducting order parameter. [1] J. L. Sarrao \textit{et al.}, Nature \underline {420}, 297 (2002); N. J. Curro et al., Nature \underline {434}, 622 (2005).