Impact ionization in GaAs: a screened exchange density functional approach

Results are presented of a fully ab-initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculated impact ionization rates show a marked orientation dependence in {\bf k} space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab-initio calculations of pair production rates in new and more complex materials.Comment: 11 pages, 4 figures, Submitted to Phys. Rev.

Impact ionization in GaAs: A screened exchange density-functional approach

Results are presented of a fully ab initio calculation of impact ionization rates in GaAs within the density functional theory framework, using a screened-exchange formalism and the highly precise all-electron full-potential linearized augmented plane wave method. The calculated impact ionization rates show a marked orientation dependence in k space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab initio calculations of pair production rates in new and more complex materials

Cosmology at the Millennium

One hundred years ago we did not know how stars generate energy, the age of the Universe was thought to be only millions of years, and our Milky Way galaxy was the only galaxy known. Today, we know that we live in an evolving and expanding Universe comprising billions of galaxies, all held together by dark matter. With the hot big-bang model, we can trace the evolution of the Universe from the hot soup of quarks and leptons that existed a fraction of a second after the beginning to the formation of galaxies a few billion years later, and finally to the Universe we see today 13 billion years after the big bang, with its clusters of galaxies, superclusters, voids, and great walls. The attractive force of gravity acting on tiny primeval inhomogeneities in the distribution of matter gave rise to all the structure seen today. A paradigm based upon deep connections between cosmology and elementary particle physics -- inflation + cold dark matter -- holds the promise of extending our understanding to an even more fundamental level and much earlier times, as well as shedding light on the unification of the forces and particles of nature. As we enter the 21st century, a flood of observations is testing this paradigm.Comment: 44 pages LaTeX with 14 eps figures. To be published in the Centennial Volume of Reviews of Modern Physic

Updated Nucleosynthesis Constraints on Unstable Relic Particles

We revisit the upper limits on the abundance of unstable massive relic particles provided by the success of Big-Bang Nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, he3, he4, li6 and li7. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of he4 and li6 exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ~100 GeV, their primordial abundance should have been below about 10^{-13} of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10^7 GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.Comment: 40 pages LaTeX, 18 eps figure

Thermionic converter with differentially heated cesium-oxygen source and method of operation

A thermionic converter having an emitter, a collector, and a source of cesium vapor is provided, wherein the source of cesium vapor is differentially heated so that said source has a hotter end and a cooler end, with cesium vapor evaporating from said hotter end into the space between the emitter and the collector and with cesium vapor condensing at said cooler end. The condensed cesium vapor migrates through a porous element from the cooler end to the hotter end

The embrittling/strengthening effects of hydrogen, boron, and phosphorus on a {Sigma}5 nickel grain boundary

The embrittling/strengthening effects of hydrogen, boron, and phosphorus on a {Sigma}5(21O) [100]nickel grain boundary are investigated by means of the full-potential linearized augmented plane wave (FLAPW) method with the generalized gradient approximation (GGA) formula. Optimized geometries for both the free surface and grain boundary systems are obtained by atomic force calculations. The results obtained show that hydrogen and phosphorus are embrittlers and that boron acts as a cohesion enhancer. An analysis of the atomic, electronic, and magnetic structures indicates that atomic size and the bonding behavior of the impurity with the surrounding nickel atoms play important roles in determining its relative embrittling or cohesion enhancing behavior

Characterization of Stress Response in a Sulfate Reducer/Methanogen Coculture

Sulfate-reducing bacteria and methanogens are found to coexist in a variety of anoxic marine sediments. In these systems they either compete for substrates or engage in successful syntrophic relationships. In our experimental setup, Desulfovibrio vulgaris Hildenborough ferments lactate, producing acetate and hydrogen. Methanococcus maripaludis, a hydrogenotrophic methanogen, then utilizes hydrogen while also incorporating limited amounts of acetate as a carbon source. Mid-log growth phase of this co-culture is achieved in 3 days growing at 37oC at which point, nearly 50percent of the initial lactate was depleted. In this study we investigate the stress response of this coculture and compare it to the D. vulgaris monoculture. Minimum Inhibitory Concentration (MIC) determinations of two environmentally relevant stressors (NO3- and NaCl) on the coculture and monoculture suggest nitrate predominantly affects M. maripaludis with a MIC of 25mM while sodium stress affects D. vulgaris with a MIC of 100mM. The response of the coculture to stressors like nitrate, nitrite, salt and peroxide was monitored by several methods. The fate of metabolites was tracked in the cultures and rates of gas evolution/utilization were measured with the Micro-Oxymax. Total biomass was measured over time with direct cell counts (including ratios of SRB: methanogen), cell protein and optical density. Metal reducing capability of log phase co-culture under NO3 stress was investigated and compared to that of under NaCl stress. Phenotype Microarray substrate utilization profiles generated by the Omnilog technology for a variety of metabolic substrates showed differential profiles for the coculture and the monoculture. Whole-genome transcriptional analysis of NaCl stressed coculture indicates up-regulation of genes coding for numerous transmembrane electron transfer enzymes

Characterization of Stress Response in a Sulfate Reducer/Methanogen Coculture

Sulfate-reducing bacteria and methanogens are found to coexist in a variety of anoxic marine sediments. In these systems they either compete for substrates or engage in successful syntrophic relationships. In our experimental setup, Desulfovibrio vulgaris Hildenborough ferments lactate, producing acetate and hydrogen. Methanococcus maripaludis, a hydrogenotrophic methanogen, then utilizes hydrogen while also incorporating limited amounts of acetate as a carbon source. Mid-log growth phase of this co-culture is achieved in 3 days growing at 37oC at which point, nearly 50percent of the initial lactate was depleted. In this study we investigate the stress response of this coculture and compare it to the D. vulgaris monoculture. Minimum Inhibitory Concentration (MIC) determinations of two environmentally relevant stressors (NO3- and NaCl) on the coculture and monoculture suggest nitrate predominantly affects M. maripaludis with a MIC of 25mM while sodium stress affects D. vulgaris with a MIC of 100mM. The response of the coculture to stressors like nitrate, nitrite, salt and peroxide was monitored by several methods. The fate of metabolites was tracked in the cultures and rates of gas evolution/utilization were measured with the Micro-Oxymax. Total biomass was measured over time with direct cell counts (including ratios of SRB: methanogen), cell protein and optical density. Metal reducing capability of log phase co-culture under NO3 stress was investigated and compared to that of under NaCl stress. Phenotype Microarray substrate utilization profiles generated by the Omnilog technology for a variety of metabolic substrates showed differential profiles for the coculture and the monoculture. Whole-genome transcriptional analysis of NaCl stressed coculture indicates up-regulation of genes coding for numerous transmembrane electron transfer enzymes