Intrinsically localized modes in uranium and the prospect for finding them in plutonium

Dynamic nonlinear localization, an emerging new area of materials physics, has the potential to fundamentally change our understanding of materials properties. These intrinsically localized modes (ILM) have been found forming in uranium above about 450 K. Comparisons with data from the literature shows that these ILMs influence many properties, including heat capacity, thermal transport, thermal expansion, and mechanical deformation. The existence of ILMs helps to explain many anomalies in uranium, some of which have been known for more than fifty years. Here, the possibility that ILMs may also play a role in the properties of plutonium is considered. The mechanism helps to explain the success of an "Invar-like" two-level model for the thermal expansion of Pu without the need to invoke local magnetic states, which have been ruled out experimentally. It also helps to explain an excitation observed in δplutonium that has energy consistent with the two-level model, but momentum dependence consistent with lattice vibrations. This excitation, energy and momentum dependence, is consistent with the non-equilibrium generation of ILMs and the thermal excitation properties of these ILMs are consistent with a two-level model. High-temperature inelastic x-rays scattering measurements of the phonon dispersion curves of ö-plutonium are needed to test this hypothesis.