Localized 5f electrons in superconducting PuCoIn5: consequences for superconductivity in PuCoGa5

The physical properties of the first In analog of the PuMGa5 (M = Co, Rh) family of superconductors, PuCoIn5, are reported. With its unit cell volume being 28% larger than that of PuCoGa5, the characteristic spin-fluctuation energy scale of PuCoIn5 is three to four times smaller than that of PuCoGa5, which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa5. This raises the possibility that the high superconducting transition temperature Tc = 18.5 K of PuCoGa5 stems from the proximity to a valence instability, while the superconductivity at Tc = 2.5 K of PuCoIn5 is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.