Invited Paper AC+AS+MI-WeM1
The Valence-Fluctuating Ground-State of δ-­Pu

Wednesday, October 21, 2015, 8:00 am, Room 230A

Plutonium (Pu) is arguably the most complex elemental metal known because its 5f 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 α-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 suggest that the electronic ground state of δ-Pu is a quantum-mechanical admixture of localized and itinerant valence configurations. The question whether the ground state of δ-Pu is indeed a true quantum-mechanical superposition may only be answered via observation of the associated virtual valence (charge) fluctuations among the distinct 5f⁴, 5f⁵, and 5f⁶ configurations. The characteristic energy scale for the associated spin fluctuations is expected to T_K= 800 K (E_sf ≈ 70 meV) that will result in a dynamical spectral response centered at this energy for T < T_K. We have performed high-energy inelastic neutron spectroscopy at room temperature using a large polycrystalline sample of δ-²⁴²Pu with a total mass of m ≈ 21 g at the Lujan Center and at the Spallation Neutron Source. Our measurements demonstrate the existence of high energy magnetic fluctuations centered at E_sf = 84 meV, in good agreement with the DMFT calculations. In addition, they allow us to extract the magnetic form factor of δ-Pu, yielding critical information about its valence state. These unprecedented results place show that the magnetism in Pu is not "missing" but dynamic, but dynamic, and is driven by virtual valence fluctuations. Our measurements provide a straightforward interpretation of the microscopic origin of the large, Pauli-like magnetic susceptibility of δ-Pu and associated Sommerfeld coefficient. Furthermore, because the various valence configurations imply distinct sizes of the Pu ion, the valence-fluctuating ground state of Pu also provides a natural explanation for its complex structural properties and in particular the large sensitivity of its volume to small changes in temperature, pressure or doping.