  Actinide materials play a special role in condensed matter physics, spanning
behaviours of itinerant d-electron and localized 4f-electron materials. An
intermediate state, found notably in Pu-based materials whose 5f electrons are
neither fully localized nor itinerant, is particularly challenging to
understand. Superconductivity appearing in some actinide materials provides
clues to the nature of the 5f electrons. PuCoGa5, the first Pu-based
superconductor, is superconducting at Tc=18.5 K. This relatively high Tc is
unprecedented in any other actinide system but is typical of itinerant electron
compounds in which superconductivity is mediated by phonons. Recent studies of
PuCoGa5 show that its superconductivity is not phonon-mediated; rather, these
experiments are consistent with superconductivity produced by antiferromagnetic
fluctuations of nearly localized 5f electrons. Similarities of PuCoGa5 with the
superconducting and normal states of isostructural 4f analogues CeMIn5 (M=Co,
Rh, Ir) and high-Tc cuprates enable new perspectives on the 5f electrons of Pu.
