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Quasiparticle excitations in PuCoGa$$_5$$ probed by muon spin rotation

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

The discovery of the first Pu-based superconductor PuCoGa$$_{5}$$ ($$T_c$$ = 18.5 K) demonstrates the rich and complex nature of Pu-based materials. The mechanism of the superconductivity in PuCoGa$$_{5}$$ is still under investigation, though recent experiments and theory suggest a magnetic origin. For example, the electronic structure of this material was investigated theoretically, and spin-fluctuation mediated superconductivity was suggested. NMR results also suggest that the superconducting order parameter of this Pu-based superconductor has d-wave symmetry. We note that Pu is a radioactive nucleus ($$^{239}$$Pu, $$T^{1/2}$$ = 24000 years), thus PuCoGa$$_{5}$$ suffers self-induced radiation damage. Previous $$mu$$SR results performed at 600 Oe in a 25-day old single crystalline sample show a linear temperature dependence for the muon spin relaxation rate $$sigma$$ ($$propto lambda^{-2}$$, $$lambda$$: magnetic penetration depth) for $$T/T_cle$$ 0.5, suggesting d-wave pairing symmetry. We have performed $$mu$$SR experimentes on the same sample after 400 days of aging in order to obtain more detailed information on the order parameter, i.e., T-dependence of $$lambda$$ at several magnetic fields, as well as the effects of aging. We found that Tc decreased from 18.5 K to 15 K for the 25-day and 400-day samples, respectively, yet the linear temperature dependence remains, consistent with a line of nodes (and suggesting d-wave pairing symmetry). These results indicate that the temperature dependence of $$lambda$$ does not depend on accumulated radiation damage over the span of time measured.

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