Pu is arguably the most complex elemental metal known because its 5 f electrons are tenuously poised at the edge between localized and itinerant configurations. This complex electronic structure leads to emergent behavior-all a direct consequence of its 5f electrons-including six allotropic phases, large volumetric changes associated with these transitions of up to 25%, and mechanical properties ranging from brittle δ-Pu to ductile δ-Pu. Pu also exhibits a Pauli-like magnetic susceptibility, electrical resistivity and a Sommerfeld coefficient of the specific heat that are an order of magnitude larger than in any other elemental metal. Finally, while experiments find no sign for static magnetism in Pu, most theories that use the correct volume predict a magnetically ordered state. This discrepancy might be reconciled by recent Dynamical Mean Field Theory (DMFT) calculations that indicate that the electronic configuration of δ-Pu is a quantum mechanical mixture of the fully localized 5f~5 (Pu~(3+)) and the 5f~ 6 (Pu~(2+)) configuration, resulting in a predicted valence in the range v = 5.2 - 5.4 confirmed by recent resonance x-ray emission spectroscopy experiments. The characteristic energy scale for the associated spin fluctuations is expected to be T_K = 800 K (≈ 70 meV) that will result in a dynamical spectral response centered at this energy for T ＜ T_K.
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