Probing the Nature of Superconductivity in the Heavy Fermion PuMGa$_{5}$ and PuMIn$_{5}$ (M$=$Co, Rh) Compounds

The discovery of superconductivity in PuCoGa$_{5}$ with a T$_{\mathrm{c}} =$ 18.5 K has generated renewed interest in Pu-based compounds. PuCoGa$_{5}$, and its superconducting cousin PuRhGa$_{5}$ (T$_{\mathrm{c}} =$ 8.7 K), have the same crystal structure as the tetragonal CeMIn$_{5}$ (T$=$Co, Rh, Ir) heavy fermion superconductors, suggesting that the structure plays a key role in generating superconductivity in these materials. While a variety of measurements have firmly established that the CeTIn$_{5}$ compounds are unconventional $d$-wave superconductors, most probably mediated by antiferromagnetic spin fluctuations, it is less clear what drives the high transition temperature in PuCoGa$_{5}$, which is an order of magnitude larger than all other know Ce- or U-based heavy fermion superconductors. The physical properties of two new members of this ``115'' family of superconductors, PuRhIn$_{5}$ and PuCoIn$_{5}$, indicate that they reside close to an antiferromagnetic quantum critical point, while the smaller effective masses and much smaller unit cell volumes of PuCoGa$_{5}$ and PuRhGa$_{5}$ suggest that they may be near a T$=$0 valence instability or that the Pu 5f electrons couple to conduction electrons in multiple channels to form ``composite'' superconducting pairs. The nature of superconductivity in these four Pu115 materials will be discussed.