Electronic Correlation and Transport Properties of Nuclear Fuel Materials

Actinide elements, such as uranium and plutonium, and their compounds are best known as nuclear materials. When engineering optimal fuel materials for nuclear power, important thermophysical properties to be considered are melting point and thermal conductivity. Understanding the physics underlying transport phenomena due to electrons and lattice vibrations in actinide systems is a crucial step toward the design of better fuels. Using first principle LDA+DMFT method, we conduct a systematic study on the correlated electronic structures and transport properties of select actinide carbides, nitrides, and oxides, many of which are nuclear fuel materials. We find that different mechanisms, electrons--electron and electron--phonon interactions, are responsible for the transport in the uranium nitride and carbide, the best two fuel materials due to their excellent thermophysical properties. Our findings allow us to make predictions on how to improve their thermal conductivities.Comment: Main article: 5 pages, 3 figures. Supplementary info: 2 pages, 1 figur

Looking at a soliton through the prism of optical supercontinuum

A traditional view on solitons in optical fibers as robust particle-like structures suited for informa- tion transmission has been significantly altered and broadened over the past decade, when solitons have been found to play the major role in generation of octave broad supercontinuum spectra in photonic-crystal and other types of optical fibers. This remarkable spectral broadening is achieved through complex processes of dispersive radiation being scattered from, emitted and transformed by solitons. Thus solitons have emerged as the major players in nonlinear frequency conversion in optical fibers. Unexpected analogies of these processes have been found with dynamics of ultracold atoms and ocean waves. This colloquium focuses on recent understanding and new insights into physics of soliton-radiation interaction and supercontinuum generation.Comment: http://rmp.aps.org/abstract/RMP/v82/i2/p1287_1 (some figures have been deleted due to space limits imposed by archive

Annealing Effect for Supersolid Fraction in 4^4He

We report on experimental confirmation of the non-classical rotational inertia (NCRI) in solid helium samples originally reported by Kim and Chan. The onset of NCRI was observed at temperatures below ~400 mK. The ac velocity for initiation of the NCRI suppression is estimated to be ~10 $\mu$m/sec. After an additional annealing of the sample at $T= 1.8$ K for 12 hours, ~ 10% relative increase of NCRI fraction was observed. Then after repeated annealing with the same conditions, the NCRI fraction was saturated. It differs from Reppy's observation on a low pressure solid sample.Comment: to be published in J. of Low Temp. Phys. (QFS2006 proceedings

Thermal conductivity vs depth profiling using the hot disk technique-Analysis of anisotropic, inhomogeneous structures

A recently developed method for analyzing the thermal conductivity vs depth variation near a sample surface has been extended to include inhomogeneous samples with anisotropy. If not considered, the anisotropy ratio in the sample structure can distort the depth-position data of the original test method. The anisotropy ratio is introduced in the original computational scheme in order to improve the depth-position estimations for inhomogeneous structures with anisotropy. The proposed approach has been tested in experiments and shown to improve depth position mapping

Effects of Ca2+ on supramolecular aggregation of natural organic matter in aqueous solutions: A comparison of molecular modeling approaches

Natural organic matter (NOM) represents a complex molecular system that cannot be fully characterized compositionally or structurally in full atomistic detail. This makes the application of molecular modeling approaches very difficult and significantly hinders quantitative investigation of NOM properties and behavior by these otherwise very efficient computational techniques. Here we report and analyze three molecular dynamics (MD) simulations of Ca2+ complexation with NOM in aqueous solutions in an attempt to quantitatively assess possible effects of model- and system size-dependence in such simulations. Despite some obvious variations in the computed results that depend on the size of the simulated system and on the parameters of the force field models used, all three simulations are quite robust and consistent. They show Ca2+ ions associated with 35-50% of the NOM carboxylic groups at near-neutral pH and point to a strong preference for the stability of bidentate-coordinated contact ion pairs. The degree and potential mechanisms of NOM supramolecular aggregation in the presence of Ca2+ ions in solution are also assessed on a semi-quantitative level from two larger-scale MD simulations

Offshore transport of shelf waters through interaction of vortices with a shelfbreak current

Author Posting. © American Meteorological Society, 2013. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 43 (2013): 905–919, doi:10.1175/JPO-D-12-0150.1.Interactions between vortices and a shelfbreak current are investigated, with particular attention to the exchange of waters between the continental shelf and slope. The nonlinear, three-dimensional interaction between an anticyclonic vortex and the shelfbreak current is studied in the laboratory while varying the ratio ε of the maximum azimuthal velocity in the vortex to the maximum alongshelf velocity in the shelfbreak current. Strong interactions between the shelfbreak current and the vortex are observed when ε > 1; weak interactions are found when ε < 1. When the anticyclonic vortex comes in contact with the shelfbreak front during a strong interaction, a streamer of shelf water is drawn offshore and wraps anticyclonically around the vortex. Measurements of the offshore transport and identification of the particle trajectories in the shelfbreak current drawn offshore from the vortex allow quantification of the fraction of the shelfbreak current that is deflected onto the slope; this fraction increases for increasing values of ε. Experimental results in the laboratory are strikingly similar to results obtained from observations in the Middle Atlantic Bight (MAB); after proper scaling, measurements of offshore transport and offshore displacement of shelf water for vortices in the MAB that span a range of values of ε agree well with laboratory predictions.Laboratory work was supported by the National Science Foundation through Grant OCE- 0081756. Glider observations in March–April 2006 were supported by the National Science Foundation through Grant OCE-0220769. Glider observations in July– October 2007 were supported by a grant from Raytheon. RET was supported by the Postdoctoral Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the Cooperative Institute for the North Atlantic Region. The REMUS observations were funded by the Office of Naval Research. GGG was supported by the National Science Foundation through Grant OCE-1129125 for analysis and writing.2013-11-0

Electronic Band Structure In A Periodic Magnetic Field

We analyze the energy band structure of a two-dimensional electron gas in a periodic magnetic field of a longitudinal antiferromagnet by considering a simple exactly solvable model. Two types of states appear: with a finite and infinitesimal longitudinal mobility. Both types of states are present at a generic Fermi surface. The system exhibits a transition to an insulating regime with respect to the longitudinal current, if the electron density is sufficiently low.Comment: 8 pages, 5 figures; to appear in Phys. Rev. B '9