9,644 research outputs found
Integral relations and the adiabatic expansion method for 1+2 reactions above the breakup threshold: Helium trimers with soft-core potentials
The integral relations formalism introduced in \cite{bar09,rom11}, and
designed to describe 1+ reactions, is extended here to collision energies
above the threshold for the target breakup. These two relations are completely
general, and in this work they are used together with the adiabatic expansion
method for the description of 1+2 reactions. The neutron-deuteron breakup, for
which benchmark calculations are available, is taken as a test of the method.
The s-wave collision between the He atom and He dimer above the
breakup threshold and the possibility of using soft-core two-body potentials
plus a short-range three-body force will be investigated. Comparisons to
previous calculations for the three-body recombination and collision
dissociation rates will be shown.Comment: To be published in Physical Review
On the rise and fall of networked societies
We review recent results on the dynamics of social networks which suggest
that the interplay between the network formation process and volatility may
lead to the occurrence of discontinuous phase transitions and phase coexistence
in a large class of models. We then investigate the effects of negative links
-- links inhibiting local growth of the network -- and of a geographical
distribution of the agents in such models. We show, by extensive numerical
simulations, that both effects enhance this phenomenology, i.e. it increases
the size of the coexistence region.Comment: 6 pages, 4 figures, Proceedings of Granada Workshop 200
Self-organized evolution in socio-economic environments
We propose a general scenario to analyze social and economic changes in
modern environments. We illustrate the ideas with a model that incorporating
the main trends is simple enough to extract analytical results and, at the same
time, sufficiently complex to display a rich dynamic behavior. Our study shows
that there exists a macroscopic observable that is maximized in a regime where
the system is critical, in the sense that the distribution of events follow
power-laws. Computer simulations show that, in addition, the system always
self-organizes to achieve the optimal performance in the stationary state.Comment: 4 pages RevTeX; needs epsf.sty and rotate.sty; submitted to Phys Rev
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General integral relations for the description of scattering states using the hyperspherical adiabatic basis
In this work we investigate 1+2 reactions within the framework of the
hyperspherical adiabatic expansion method. To this aim two integral relations,
derived from the Kohn variational principle, are used. A detailed derivation of
these relations is shown. The expressions derived are general, not restricted
to relative partial waves, and with applicability in multichannel
reactions. The convergence of the -matrix in terms of the adiabatic
potentials is investigated. Together with a simple model case used as a test
for the method, we show results for the collision of a He atom on a \dimer
dimer (only the elastic channel open), and for collisions involving a Li
and two He atoms (two channels open).Comment: Accepted for publication in Physical Review
Variational description of continuum states in terms of integral relations
Two integral relations derived from the Kohn Variational Principle (KVP) are
used for describing scattering states. In usual applications the KVP requires
the explicit form of the asymptotic behavior of the scattering wave function.
This is not the case when the integral relations are applied since, due to
their short range nature, the only condition for the scattering wave function
is that it be the solution of in the internal region.
Several examples are analyzed for the computation of phase-shifts from bound
state type wave functions or, in the case of the scattering of charged
particles, it is possible to obtain phase-shifts using free asymptotic
conditions. As a final example we discuss the use of the integral relations in
the case of the Hyperspherical Adiabatic method.Comment: 34 pages, 7 figures, accepted in Phys. Rev.
Theoretical studies of Si and GaAs surfaces and initial steps in the oxidation
Using ab initio quantum chemical methods (generalized valence bond), we examine (i) the electronic states of Si (111) and GaAs (110) surface, (ii) the relaxation of the Si (111) surface, (iii) the reconstruction of the GaAs surface, (iv) the initial steps in the chemisorption of O_2 on Si (111), and (v) the bonding of O atom to Ga and As centers
Sputniks cubanos. De cómo la URSS ocupó la imaginación de una generación
Resumen: A pesar de la cierta marginalidad de la ciencia ficción en Cuba, ésta no estuvo exenta de la influencia cultural que sufrió la isla al sovietizarse. De la URSS y el entorno comunista llegaron cientos de obras del género, al mismo tiempo que la producción local se reducía a cero en los años setenta. Los autores cubanos tomaron partido, unos defendieron los nuevos estilos e ideas venidos del Este; otros decidieron seguir su camino, algunas veces, a contracorriente. Despite the somewhat marginalization of science fiction in Cuba, the genre could not avoid the cultural influence that the island experienced when it adopted Soviet cannons. From the USSR and the Communist allies came hundreds of genre publications in the seventies, while local production was reduced to zero. Most Cuban writers took sides, some defended the new styles and ideas that came from the East; others decided not to, and even created sometimes a counter discourse
Advances in solid polymer electrolyte fuel cell technology with low-platinum-loading electrodes
The Gemini Space program demonstrated the first major application of fuel cell systems. Solid polymer electrolyte fuel cells were used as auxiliary power sources in the spacecraft. There has been considerable progress in this technology since then, particularly with the substitution of Nafion for the polystyrene sulfonate membrane as the electrolyte. Until recently the performance was good only with high platinum loading (4 mg/sq cm) electrodes. Methods are presented to advance the technology by (1) use of low platinum loading (0.35 mg/sq cm) electrodes; (2) optimization of anode/membrane/cathode interfaces by hot pressing; (3) pressurization of reactant gases, which is most important when air is used as cathodic reactant; and (4) adequate humidification of reactant gases to overcome the water management problem. The high performance of the fuel cell with the low loading of platinum appears to be due to the extension of the three dimensional reaction zone by introduction of a proton conductor, Nafion. This was confirmed by cyclic voltammetry
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