Unconventional Spin-Fluctuation Mediated Superconductivity in PuMGa$_5$ (M=Co, Rh)

The discovery of superconductivity at $T_c$=18.5 K in PuCoGa$_5$ and at $T_c=9$ K in PuRhGa$_5$ has generated renewed interest in Pu-based intermetallic compounds. The thermodynamic properties of PuMGa$_5$ (M=Co, Rh), such as the initial slope of the upper critical field and specific heat jump at $T_c$, are consistent with an electronic specific heat coefficient $\gamma \sim$ 50-100 mJ/mol-K$^2$ indicating moderately heavy fermion behavior in these materials. The normal and superconducting states of these two Pu-based superconductors are remarkably similar to those of the well-known heavy-fermion CeMIn$_5$ (M=Co, Rh, Ir) superconductors, in which superconductivity is mediated by antiferromagnetic spin fluctuations. We present electrical resistivity and nuclear spin lattice relaxation measurements on PuMGa$_5$ that indicates a single (spin-fluctuation) energy scale dominates the physical properties, suggesting that spin fluctuations control both the superconducting and normal states. We contrast these behaviors with those observed in isostructural UMGa$_5$ and NpMGa$_5$.