Electronic structure calculations combining the local-density approximation with an exact diagonalization of the Anderson impurity model show an intermediate 5$f^5$-5$f^6$-valence ground state and delocalization of the 5$f^5$ multiplet of the Pu atom 5$f$ shell in PuCoIn${}_5$, PuCoGa${}_5$, and $\delta$-Pu. The 5$f$ local magnetic moment is compensated by a moment formed in the surrounding cloud of conduction electrons. For PuCoGa${}_5$ and $\delta$-Pu the compensation is complete, and the Anderson impurity ground state is a singlet. For PuCoIn${}_5$ the compensation is partial, and the Pu ground state is magnetic. We suggest that the unconventional $d$-wave superconductivity is likely mediated by the 5$f$-state antiferromagnetic fluctuations in PuCoIn${}_5$ and by valence fluctuations in PuCoGa${}_5$.

• Received 29 October 2012

DOI:https://doi.org/10.1103/PhysRevB.87.020505