958 research outputs found
Relativistic Mean Field Calculations of and Hypernuclei
Single--particle spectra of and hypernuclei are
calculated within a relativistic mean--field theory. The hyperon couplings used
are compatible with the binding in saturated nuclear matter,
neutron-star masses and experimental data on levels in hypernuclei.
Special attention is devoted to the spin-orbit potential for the hyperons and
the influence of the -meson field (isospin dependent interaction).Comment: 18 pages, including 2 figs., figs. 1 and 4-6 available as
postscript-datasets on request; written in Latex, report# LBL-3303
Standard and Non-Standard Plasma Neutrino Emission Revisited
On the basis of Braaten and Segel's representation of the electromagnetic
dispersion relations in a QED plasma we check the numerical accuracy of several
published analytic approximations to the plasma neutrino emission rates. As we
find none of them satisfactory we derive a new analytic approximation which is
accurate to within 4\%\ where the plasma process dominates. The correct
emission rates in the parameter regime relevant for the red giant branch in
globular clusters are larger by about \% than those of previous stellar
evolution calculations. Therefore, the core mass of red giants at the He flash
is larger by about 0.005\M_\odot or 1\% than previously thought. Our bounds
on neutrino magnetic dipole moments remain virtually unchanged.Comment: LaTeX, 16 pages, 12 figures on request from authors, MPI-Ph/93-6
Impurity-enhanced Aharonov-Bohm effect in neutral quantum-ring magnetoexcitons
We study the role of impurity scattering on the photoluminescence (PL)
emission of polarized magnetoexcitons. We consider systems where both the
electron and hole are confined on a ring structure (quantum rings) as well as
on a type-II quantum dot. Despite their neutral character, excitons exhibit
strong modulation of energy and oscillator strength in the presence of magnetic
fields. Scattering impurities enhance the PL intensity on otherwise "dark"
magnetic field windows and non-zero PL emission appears for a wide magnetic
field range even at zero temperature. For higher temperatures, impurity-induced
anticrossings on the excitonic spectrum lead to unexpected peaks and valleys on
the PL intensity as function of magnetic field. Such behavior is absent on
ideal systems and can account for prominent features in recent experimental
results.Comment: 7 pages, 7 figures, RevTe
Quantum Fields a la Sylvester and Witt
A structural explanation of the coupling constants in the standard model, i.e
the fine structure constant and the Weinberg angle, and of the gauge fixing
contributions is given in terms of symmetries and representation theory. The
coupling constants are normalizations of Lorentz invariantly embedded little
groups (spin and polarization) arising in a harmonic analysis of quantum vector
fields. It is shown that the harmonic analysis of massless fields requires an
extension of the familiar Fourier decomposition, containing also indefinite
unitary nondecomposable time representations. This is illustrated by the
nonprobabilistic contributions in the electromagnetic field.Comment: 18 pages LaTeX file (62 kB), all macros are include
Coulomb interaction effects on the electronic structure of radial polarized excitons in nanorings
The electronic structure of radially polarized excitons in structured
nanorings is analyzed, with emphasis in the ground-state properties and their
dependence under applied magnetic fields perpendicular to the ring plane. The
electron-hole Coulomb attraction has been treated rigorously, through numerical
diagonalization of the full exciton Hamiltonian in the non-interacting
electron-hole pairs basis. Depending on the relative weight of the kinetic
energy and Coulomb contributions, the ground-state of polarized excitons has
"extended" or "localized" features. In the first case, corresponding to small
rings dominated by the kinetic energy, the ground-state shows Aharonov-Bohm
(AB) oscillations due to the individual orbits of the building particles of the
exciton. In the localized regime, corresponding to large rings dominated by the
Coulomb interaction, the only remaining AB oscillations are due to the magnetic
flux trapped between the electron and hole orbits. This dependence of the
exciton, a neutral excitation, on the flux difference confirms this feature as
a signature of Coulomb dominated polarized excitons. Analytical approximations
are provided in both regimens, which accurate reproduce the numerical results.Comment: 9 pages, including 6 figure
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Reversible Conductive Inkjet Printing of Healable and Recyclable Electrodes on Cardboard and Paper
Conductive inkjet printing with metal nanoparticles is irreversible because the particles are sintered into a continuous metal film. The resulting structures are difficult to remove or repair and prone to cracking. Here, a hybrid ink is used to obviate the sintering step and print interconnected particle networks that become highly conductive immediately after drying. It is shown that reversible conductive printing is possible on low-cost cardboard samples after applying standard paper industry coats that are adapted in terms of surface energy and porosity. The conductivity of the printed films approaches that of sintered standard inks on the same substrate, but the mobility of the hybrid particle film makes them less sensitive to cracks during bending and folding of the substrate. Damages that occur can be partially repaired by wetting the film such that particle mobility is increased and particles move to bridge insulating gaps in the film. It is demonstrated that the conductive material can be recovered from the cardboard at the end of its life time and be redispersed to recycle the particles and reuse them in conductive inks
Bioinformatic evidence for a widely distributed, ribosomally produced electron carrier precursor, its maturation proteins, and its nicotinoprotein redox partners
<p>Abstract</p> <p>Background</p> <p>Enzymes in the radical SAM (rSAM) domain family serve in a wide variety of biological processes, including RNA modification, enzyme activation, bacteriocin core peptide maturation, and cofactor biosynthesis. Evolutionary pressures and relationships to other cellular constituents impose recognizable grammars on each class of rSAM-containing system, shaping patterns in results obtained through various comparative genomics analyses.</p> <p>Results</p> <p>An uncharacterized gene cluster found in many Actinobacteria and sporadically in Firmicutes, Chloroflexi, Deltaproteobacteria, and one Archaeal plasmid contains a PqqE-like rSAM protein family that includes Rv0693 from <it>Mycobacterium tuberculosis</it>. Members occur clustered with a strikingly well-conserved small polypeptide we designate "mycofactocin," similar in size to bacteriocins and PqqA, precursor of pyrroloquinoline quinone (PQQ). Partial Phylogenetic Profiling (PPP) based on the distribution of these markers identifies the mycofactocin cluster, but also a second tier of high-scoring proteins. This tier, strikingly, is filled with up to thirty-one members per genome from three variant subfamilies that occur, one each, in three unrelated classes of nicotinoproteins. The pattern suggests these variant enzymes require not only NAD(P), but also the novel gene cluster. Further study was conducted using SIMBAL, a PPP-like tool, to search these nicotinoproteins for subsequences best correlated across multiple genomes to the presence of mycofactocin. For both the short chain dehydrogenase/reductase (SDR) and iron-containing dehydrogenase families, aligning SIMBAL's top-scoring sequences to homologous solved crystal structures shows signals centered over NAD(P)-binding sites rather than over substrate-binding or active site residues. Previous studies on some of these proteins have revealed a non-exchangeable NAD cofactor, such that enzymatic activity <it>in vitro </it>requires an artificial electron acceptor such as N,N-dimethyl-4-nitrosoaniline (NDMA) for the enzyme to cycle.</p> <p>Conclusions</p> <p>Taken together, these findings suggest that the mycofactocin precursor is modified by the Rv0693 family rSAM protein and other enzymes in its cluster. It becomes an electron carrier molecule that serves <it>in vivo </it>as NDMA and other artificial electron acceptors do <it>in vitro</it>. Subclasses from three different nicotinoprotein families show "only-if" relationships to mycofactocin because they require its presence. This framework suggests a segregated redox pool in which mycofactocin mediates communication among enzymes with non-exchangeable cofactors.</p
Accurate Profiling of Microbial Communities from Massively Parallel Sequencing using Convex Optimization
We describe the Microbial Community Reconstruction ({\bf MCR}) Problem, which
is fundamental for microbiome analysis. In this problem, the goal is to
reconstruct the identity and frequency of species comprising a microbial
community, using short sequence reads from Massively Parallel Sequencing (MPS)
data obtained for specified genomic regions. We formulate the problem
mathematically as a convex optimization problem and provide sufficient
conditions for identifiability, namely the ability to reconstruct species
identity and frequency correctly when the data size (number of reads) grows to
infinity. We discuss different metrics for assessing the quality of the
reconstructed solution, including a novel phylogenetically-aware metric based
on the Mahalanobis distance, and give upper-bounds on the reconstruction error
for a finite number of reads under different metrics. We propose a scalable
divide-and-conquer algorithm for the problem using convex optimization, which
enables us to handle large problems (with species). We show using
numerical simulations that for realistic scenarios, where the microbial
communities are sparse, our algorithm gives solutions with high accuracy, both
in terms of obtaining accurate frequency, and in terms of species phylogenetic
resolution.Comment: To appear in SPIRE 1
Life in Hot Carbon Monoxide: The Complete Genome Sequence of Carboxydothermus hydrogenoformans Z-2901
We report here the sequencing and analysis of the genome of the thermophilic bacterium Carboxydothermus hydrogenoformans Z-2901. This species is a model for studies of hydrogenogens, which are diverse bacteria and archaea that grow anaerobically utilizing carbon monoxide (CO) as their sole carbon source and water as an electron acceptor, producing carbon dioxide and hydrogen as waste products. Organisms that make use of CO do so through carbon monoxide dehydrogenase complexes. Remarkably, analysis of the genome of C. hydrogenoformans reveals the presence of at least five highly differentiated anaerobic carbon monoxide dehydrogenase complexes, which may in part explain how this species is able to grow so much more rapidly on CO than many other species. Analysis of the genome also has provided many general insights into the metabolism of this organism which should make it easier to use it as a source of biologically produced hydrogen gas. One surprising finding is the presence of many genes previously found only in sporulating species in the Firmicutes Phylum. Although this species is also a Firmicutes, it was not known to sporulate previously. Here we show that it does sporulate and because it is missing many of the genes involved in sporulation in other species, this organism may serve as a “minimal” model for sporulation studies. In addition, using phylogenetic profile analysis, we have identified many uncharacterized gene families found in all known sporulating Firmicutes, but not in any non-sporulating bacteria, including a sigma factor not known to be involved in sporulation previously
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