We investigate the nonequilibrium evolution of a scalar field in (2+1) dimensions. The field is set in a double-well potential in contact (open) or not (closed) with a heat bath. For closed systems, we observe the synchronized emergence of coherent spatiotemporal configurations, identified with oscillons. This initial global ordering degenerates into localized order until all oscillons disappear. We show that the synchronization is driven by resonant parametric oscillations of the field's zero mode and that local ordering is only possible outside equipartition. None of these orderings occur for open systems.

The availability of neutron spallation-source instruments that provide total scattering powder diffraction has led to an increased application of real-space structure analysis using the pair distribution function. Currently, the analytical treatment of finite size effects within pair distribution refinement procedures is limited. To that end, an envelope function is derived which transforms the pair distribution function of an infinite solid into that of a spherical particle with the same crystal structure. Distributions of particle sizes are then considered, and the associated envelope function is used to predict the particle size distribution of an experimental sample of gold nanoparticles from its pair distribution function alone. Finally, complementing the wealth of existing diffraction analysis, the peak broadening for the structure factor of spherical particles, expressed as a convolution derived from the envelope functions, is calculated exactly for all particle size distributions considered, and peak maxima, offsets, and asymmetries are discussed.LA-UR 05-8264

The scientifically fascinating question of the spatial extent and bonding of the 5f orbitals of Pu and its six different phases extends to its δ-retained alloys and the mechanism by which Ga and a number of other unrelated elements stabilize its low density face-centered-cubic (fcc) structure. This issue of phase stability is also important technologically because of its significance to Science-Based Stockpile Stewardship. Answering these questions requires information on the local order and structure around the Ga and its effects on the Pu. We have addressed this by characterizing the structures of a large number of Pu-Ga and two Pu-In and one Pu-Ce δ alloys, including a set of high purity δ Pu 1−x Ga x materials with 1.7 ࣘ x ࣘ 6.4 at. % Ga that span the low [Ga] portion of the δ region of the phase diagram across the ß3.3 at. % Ga metastability boundary, with extended x-ray absorption fine structure (EXAFS) spectroscopy that probes the element specific local structure, supplemented by x-ray pair distribution function analysis that gives the total local structure to longer distances, and x-ray diffraction that gives the long-range average structure of the periodic component of the materials. Detailed analyses indicate that the alloys at and below a nominal composition of ß3.3 at. % Ga are heterogeneous and in addition to the δ phase also contain up to ß20% of a novel, coexisting "σ " structure for Pu that forms in nanometer scale domains that are locally depleted in Ga. The invariance of the Ga EXAFS with composition indicates that this σ structure forms in Ga-depleted domains that result from the Ga atoms in the δ phase self-organizing into a quasi-intermetallic with a stoichiometry of Pu 25−35 Ga so that δ Pu-Ga is neither a random solid solution nor the more stable Pu 3 Ga + α. Above this 3.3 at. % Ga nominal composition, the δ Pu-Ga alloy is homogeneous, and no σ phase is present. These results that demonstrate that collective and cooperative behavior in the interactions between the alloy elements as well as local elastic forces are crucial in determining the properties of complex materials and contradict the conventional mechanism for martensitic transformations, in this case indicating that nucleation is not the rate limiting step.

We investigate the role played by fast quenching on the decay of metastable (or false vacuum) states. Instead of the exponentially slow decay rate per unit volume, Gamma(HN) approximately exp([-E(b)/k(B)T] (E(b) is the free energy of the critical bubble), predicted by homogeneous nucleation theory, we show that under fast enough quenching the decay rate is a power law Gamma(RN) approximately [E(b)/k(B)T](-B), where B is weakly sensitive to the temperature. For a range of parameters, large-amplitude oscillations about the metastable state trigger the resonant emergence of coherent subcritical configurations. Decay mechanisms for different E(b) are proposed and illustrated in a (2+1)-dimensional scalar field model.

The structural properties of the phase coexistence of chemically ordered L1(0) and chemically disordered structures within binary alloys are investigated, using the NiMn system as an example. Theoretical and numerical predictions of the signatures that one might expect in data from local and extended probes are presented in an attempt to explain the presence of antiferromagnetism in NiMn when no L1(0) signatures appear in diffraction data. Two scenarios are considered. In the first scenario, the tetragonal L1(0) structure and fcc chemically disordered structure are distributed evenly into uncorrelated domains of specified average diameter. The diffraction limit, below which the two structures can only be distinguished using a local probe, is quantified with respect to the domain diameter by applying straightforward diffraction analysis. In the second scenario, domains with chemical ordering oriented in different directions are required to maintain their atomic coherence with each other. A numerical treatment is used to illustrate the long-range strain that results from elastic energy considerations, and the effects on the structure factor (extended probe) and pair distribution function (local probe) are investigated.