15,327 research outputs found
Electron-Positron Pair Production in Space- or Time-Dependent Electric Fields
Treating the production of electron and positron pairs by a strong electric
field from the vacuum as a quantum tunneling process we derive, in
semiclassical approximation, a general expression for the pair production rate
in a -dependent electric field pointing in the -direction. We also
allow for a smoothly varying magnetic field parallel to . The result is
applied to a confined field for , a
semi-confined field for , and a linearly increasing
field . The boundary effects of the confined fields on
pair-production rates are exhibited. A simple variable change in all formulas
leads to results for electric fields depending on time rather than space.
In addition, we discuss tunneling processes in which empty atomic bound
states are spontaneously filled by negative-energy electrons from the vacuum
under positron emission. In particular, we calculate the rate at which the
atomic levels of a bare nucleus of finite size and large
are filled by spontaneous pair creation.Comment: 33 pages and 9 figures. to appear in Phys. Rev.
Statistical properties of Klauder-Perelomov coherent states for the Morse potential
We present in this paper a realistic construction of the coherent states for
the Morse potential using the Klauder-Perelomov approach . We discuss the
statistical properties of these states, by deducing the Q- and P-distribution
functions. The thermal expectations for the quantum canonical ideal gas of the
Morse oscillators are also calculated
Who Needs Credit and Who Gets Credit in Eastern Europe?
Based on survey data covering 8,387 firms in 20 countries we compare credit demand and credit supply for firms in Eastern Europe to those for firms in selected Western European countries.Credit Constraints;Banking sector;Transition economies
Phase transition for the frog model
We study a system of simple random walks on graphs, known as frog model. This
model can be described as follows: There are active and sleeping particles
living on some graph G. Each active particle performs a simple random walk with
discrete time and at each moment it may disappear with probability 1-p. When an
active particle hits a sleeping particle, the latter becomes active. Phase
transition results and asymptotic values for critical parameters are presented
for Z^d and regular trees
Evolution of isolated neutron stars in globular clusters: number of Accretors
With a simple model from the point of view of population synthesis we try to
verify an interesting suggestion made by Pfahl & Rappaport (2001) that dim
sources in globular clusters (GCs) can be isolated accreting neutron stars
(NSs). Simple estimates show, that we can expect about 0.5-1 accreting isolated
NS per typical GC with in correspondence with observations.
Properties of old accreting isolated NSs in GCs are briefly discussed. We
suggest that accreting NSs in GCs experienced significant magnetic field decay.Comment: 6 pages, no figures. Submitted to Astronomical and Astrophysical
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Formation of energy savings directions at oil and fat industry enterprises of Ukraine
In the Ukrainian economy, due to the high energy intensity of gross domestic product (2.6 times higher than the industrialized countries of the world), rising prices for traditional fuel and energy resources, increasing energy imports, the problem of energy saving in Ukraine is one of the topical and strategic value. One of the largest consumers of the fuel and energy resources of the food industry is the oil and fat industry. A generic indicator of the efficiency of energy use at the oil and fat industry is energy intensity of production, ie an indicator characterizing the total costs of fuel and energy resources for the production of a unit of finished products. The article analyzes in detail the indices of the use of fuel and energy resources as a whole for the oil and fat industry, for certain types of its products, as well as for individual enterprises of this industry. The main priority directions of energy efficiency improvement in the oil and fat industry are identified and substantiated, which the authors refer to: creation of an energy complex for burning sunflower husk with obtaining heat and electricity, granulation of husk with its further use as an alternative bio-economic entity. Specific recommendations have been developed to reduce the energy content of oil and fat products through the development of highly efficient energy-saving technologies, circuits, lines, units, units and energy-efficient equipment at industrial enterprises
BCS-BEC crossover in a system of microcavity polaritons
We investigate the thermodynamics and signatures of a polariton condensate
over a range of densities, using a model of microcavity polaritons with
internal structure. We determine a phase diagram for this system including
fluctuation corrections to the mean-field theory. At low densities the
condensation temperature, T_c, behaves like that for point bosons. At higher
densities, when T_c approaches the Rabi splitting, T_c deviates from the form
for point bosons, and instead approaches the result of a BCS-like mean-field
theory. This crossover occurs at densities much less than the Mott density. We
show that current experiments are in a density range where the phase boundary
is described by the BCS-like mean-field boundary. We investigate the influence
of inhomogeneous broadening and detuning of excitons on the phase diagram.Comment: 20 pages, 6 figure
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