178 research outputs found
Phase Structure in a Hadronic Chiral Model
We study the phase diagram of a hadronic chiral flavor-SU(3) model. Heavy
baryon resonances can induce a phase structure that matches current results
from lattice-QCD calculations at finite temperature and baryon density.
Furthermore, we determine trajectories of constant entropy per net baryon in
the phase diagram.Comment: 4 pages, 5 figure
Dense Quarks, and the Fermion Sign Problem, in a SU(N) Matrix Model
We study the effect of dense quarks in a SU(N) matrix model of deconfinement.
For three or more colors, the quark contribution to the loop potential is
complex. After adding the charge conjugate loop, the measure of the matrix
integral is real, but not positive definite. In a matrix model, quarks act like
a background Z(N) field; at nonzero density, the background field also has an
imaginary part, proportional to the imaginary part of the loop. Consequently,
while the expectation values of the loop and its complex conjugate are both
real, they are not equal. These results suggest a possible approach to the
fermion sign problem in lattice QCD.Comment: 9 pages, 3 figure
Synthesis of intermetallic hydrogen storage materials based on TI-CR by glow discharge plasma
The TiCr2 intermetallic compound with the C36 hexagonal Laves phase has been synthesized by glow discharge plasma. The hydrogen absorption-desorption properties of the obtained alloy were investigated. It is shown that hydrogen storage capacity of material was 0.43 wt % at 85 °C and 8 atm
Following carbon condensation by in-situ TEM : towards a rational understanding of the processes in the synthesis of nitrogen-doped carbonaceous materials.
Porous carbonaceous materials obtained from biomass have been an important class of CO2 sorbents since ancient times. Recent progress in carbon-based adsorbent technology is based on the implication of the concept of heteroatom doping. In this respect, the synthesis of carbonaceous materials through one-step condensation of cheap nitrogen-containing molecular precursors is an attractive strategy for obtaining such N-doped carbons. The design of the adsorbents obtained by this route relies on the careful adjustment of synthesis parameters, such as the temperature, the heating rate, and the atmosphere. However, in most cases, the latter's choice remains rather empirical due to the lack of a fundamental understanding of the condensation mechanism of molecular precursors. In this work, we followed the structural, morphological, and chemical evolution of a molecular precursor (uric acid) at the nanoscale using a combination of in-situ condensation inside a scanning transmission electron microscope with ex-situ analysis of the products of condensation at different temperatures, atmospheres, and heating rates, and correlate our findings with the CO2 sorption properties of the obtained materials. We showed that varying pressures and reaction rates result in particles with different porosity. The porosity of the surface of the particles during the early stages of condensation governs the subsequent release of volatiles and the development of a hierarchical pore structure. We found that synthesis in vacuum enables effective condensation at considerably low temperatures (500 °C). Using a higher heating rate (10 °C/min) suppresses structural ripening and preserves the optimal size of micropores, thus giving a CO2 uptake twice as high compared to samples synthesized in nitrogen atmosphere, which is commonly used, preserving the same selectivity.ER
Current Status of Quark Gluon Plasma Signals
Compelling evidence for the creation of a new form of matter has been claimed
to be found in Pb+Pb collisions at SPS. We discuss the uniqueness of often
proposed experimental signatures for quark matter formation in relativistic
heavy ion collisions. It is demonstrated that so far none of the proposed
signals like J\psi meson production/suppression, strangeness enhancement,
dileptons, and directed flow unambigiously show that a phase of deconfined
matter has been formed in SPS Pb+Pb collisions. We emphasize the need for
systematic future measurements to search for simultaneous irregularities in the
excitation functions of several observables in order to come close to pinning
the properties of hot, dense QCD matter from data.Comment: 12 pages, 6 figures, Proceedings of the Symposium on Fundamental
Issues in Elementary Matter In Honor and Memory of Michael Danos 241.
WE-Heraeus-Seminar Bad Honnef, Germany, 25--29 September 2000. To appear in
Heavy Ion Phy
Radiation chemistry provides nanoscopic insights into the role of intermediate phases in CeO<sub>2</sub> mesocrystal formation
The role of intermediate phases in CeO2 mesocrystal formation from aqueous Ce(III) solutions subjected to gamma-radiation was studied. Radiolytically formed hydroxyl radicals convert soluble Ce(III) into less soluble Ce(IV). Transmission electron microscopy (TEM) and X-ray diffraction studies of samples from different stages of the process allowed the identification of several stages in CeO2 mesocrystal evolution following the oxidation to Ce(IV): (1) formation of hydrated Ce(IV)-hydroxides, serving as intermediates in the liquid-to-solid phase transformation; (2) CeO2 primary particle growth inside the intermediate phase; (3) alignment of the primary particles into "pre-mesocrystals" and subsequently to mesocrystals, guided by confinement of the amorphous intermediate phase and accompanied by the formation of "mineral bridges". Further alignment of the obtained mesocrystals into supracrystals occurs upon slow drying, making it possible to form complex hierarchical architectures
Direct SUSY dark matter detection-Theoretical rates due to the spin
The recent WMAP data have confirmed that exotic dark matter together with the
vacuum energy (cosmological constant) dominate in the flat Universe. Thus the
direct dark matter detection, consisting of detecting the recoiling nucleus, is
central to particle physics and cosmology. Supersymmetry provides a natural
dark matter candidate, the lightest supersymmetric particle (LSP). The relevant
cross sections arise out of two mechanisms: i) The coherent mode, due to the
scalar interaction and ii) The spin contribution arising from the axial
current. In this paper we will focus on the spin contribution, which is
expected to dominate for light targets.
For both modes it is possible to obtain detectable rates, but in most models
the expected rates are much lower than the present experimental goals. So one
should exploit two characteristic signatures of the reaction, namely the
modulation effect and, in directional experiments, the correlation of the event
rates with the sun's motion.
In standard non directional experiments the modulation is small, less than
two per cent. In the case of the directional event rates we like to suggest
that the experiments exploit two features, of the process, which are
essentially independent of the SUSY model employed, namely: 1) The
forward-backward asymmetry, with respect to the sun's direction of motion, is
very large and 2) The modulation is much larger, especially if the observation
is made in a plane perpendicular to the sun's velocity. In this case the
difference between maximum and minimum can be larger than 40 per cent and the
phase of the Earth at the maximum is direction dependent.Comment: 16 Latex pages, 15 figures, 3 table
Describing transverse dynamics and space-time evolution at RHIC in a hydrodynamic model with statistical hadronization
A hydrodynamic model coupled to the statistical hadronization code
Therminator is used to study a set of observables in the soft sector at RHIC. A
satisfactory description of the pT-spectra and elliptic flow is obtained,
similarly to other hydrodynamic models. With the Gaussian initial conditions
the transverse femtoscopic radii are also reproduced, providing a possible
solution of the RHIC HBT puzzle.Comment: to appear in the conference proceedings for Quark Matter 2009, March
30 - April 4, Knoxville, Tennesse
Cu<sup>II</sup>/Cu<sup>I</sup> decorated N-doped carbonaceous electrocatalysts for the oxygen reduction reaction
The oxygen reduction reaction (ORR) that for instance takes place at the cathode of fuel cells is one of the most examined model reactions of energy conversion. The ORR presents sluggish reaction kinetics, thus limiting the overall efficiency of these cells. Pt-based catalysts are still the widest choice though they exhibit important drawbacks such as long-term instability and intolerance to methanol crossover. In this context, engineering transition metals in the form of nano- and subnano-sites on carbonaceous supports has the potential of becoming an alternative to scarce noble metal-based catalysts. Herein, we describe a simple synthetic route towards CuII/CuI decorated N-doped carbonaceous ORR electrocatalysts. CuII/CuI nanosites are obtained by calcination in air of an ionic liquid derived noble carbonaceous support impregnated with copper(ii) acetate. The strong interaction between the copper and the noble support foster the co-formation of CuII/CuI nanosites. Larger amounts of copper(ii) acetate translate into larger amounts of CuI and lower Tafel slopes. The material with 4 wt% of copper catalyzes the selective reduction of oxygen through a 4-electron transfer pathway and exhibits a lower Tafel slope than commercial platinum, a minimal overpotential, and a higher limiting current density. Moreover, all materials show promising durability and high methanol stability, which makes them promising to replace noble metals for the ORR
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