362 research outputs found
A classical Over Barrier Model to compute charge exchange between ions and one-optical-electron atoms
In this paper we study theoretically the process of electron capture between
one-optical-electron atoms (e.g. hydrogenlike or alkali atoms) and ions at
low-to-medium impact velocities (v/v_e <= 1) working on a modification of an
already developed classical Over Barrier Model (OBM) [V. Ostrovsky, J. Phys. B:
At. Mol. Opt. Phys. {\bf 28} 3901 (1995)], which allows to give a
semianalytical formula for the cross sections. The model is discussed and then
applied to a number of test cases including experimental data as well as data
coming from other sophisticated numerical simulations. It is found that the
accuracy of the model, with the suggested corrections and applied to quite
different situations, is rather high.Comment: 12 pages REVTEX, 5 EPSF figures, submitted to Phys Rev
Complete solution of a constrained tropical optimization problem with application to location analysis
We present a multidimensional optimization problem that is formulated and
solved in the tropical mathematics setting. The problem consists of minimizing
a nonlinear objective function defined on vectors over an idempotent semifield
by means of a conjugate transposition operator, subject to constraints in the
form of linear vector inequalities. A complete direct solution to the problem
under fairly general assumptions is given in a compact vector form suitable for
both further analysis and practical implementation. We apply the result to
solve a multidimensional minimax single facility location problem with
Chebyshev distance and with inequality constraints imposed on the feasible
location area.Comment: 20 pages, 3 figure
Neutron Halo Isomers in Stable Nuclei and their Possible Application for the Production of Low Energy, Pulsed, Polarized Neutron Beams of High Intensity and High Brilliance
We propose to search for neutron halo isomers populated via -capture
in stable nuclei with mass numbers of about A=140-180 or A=40-60, where the
or neutron shell model state reaches zero binding energy.
These halo nuclei can be produced for the first time with new -beams of
high intensity and small band width ( 0.1%) achievable via Compton
back-scattering off brilliant electron beams thus offering a promising
perspective to selectively populate these isomers with small separation
energies of 1 eV to a few keV. Similar to single-neutron halo states for very
light, extremely neutron-rich, radioactive nuclei
\cite{hansen95,tanihata96,aumann00}, the low neutron separation energy and
short-range nuclear force allows the neutron to tunnel far out into free space
much beyond the nuclear core radius. This results in prolonged half lives of
the isomers for the -decay back to the ground state in the 100
ps-s range. Similar to the treatment of photodisintegration of the
deuteron, the neutron release from the neutron halo isomer via a second,
low-energy, intense photon beam has a known much larger cross section with a
typical energy threshold behavior. In the second step, the neutrons can be
released as a low-energy, pulsed, polarized neutron beam of high intensity and
high brilliance, possibly being much superior to presently existing beams from
reactors or spallation neutron sources.Comment: accepted for publication in Applied Physics
Finite size and temperature effects in the AF Heisenberg model
The low temperature and large volume effects in the d=2+1 antiferromagnetic
quantum Heisenberg model are dominated by magnon excitations. The leading and
next-to-leading corrections are fully controlled by three physical constants,
the spin stiffness, the spin wave velocity and the staggered magnetization.
Among others, the free energy, the ground state energy, the low lying
excitations, staggered magnetization, staggered and uniform susceptibilities
are studied here. The special limits of very low temperature and infinite
volume are considered also.Comment: 44 pages, LATEX, no figure
Two-domains bulklike Fermi surface of Ag films deposited onto Si(111)-(7x7)
Thick metallic silver films have been deposited onto Si(111)-(7x7) substrates
at room temperature. Their electronic properties have been studied by using
angle resolved photoelectron spectroscopy (ARPES). In addition to the
electronic band dispersion along the high-symmetry directions, the Fermi
surface topology of the grown films has been investigated. Using ARPES, the
spectral weight distribution at the Fermi level throughout large portions of
the reciprocal space has been determined at particular perpendicular
electron-momentum values. Systematically, the contours of the Fermi surface of
these films reflected a sixfold symmetry instead of the threefold symmetry of
Ag single crystal. This loss of symmetry has been attributed to the fact that
these films appear to be composed by two sets of domains rotated 60 from
each other. Extra, photoemission features at the Fermi level were also
detected, which have been attributed to the presence of surface states and
\textit{sp}-quantum states. The dimensionality of the Fermi surface of these
films has been analyzed studying the dependence of the Fermi surface contours
with the incident photon energy. The behavior of these contours measured at
particular points along the Ag L high-symmetry direction puts forward
the three-dimensional character of the electronic structure of the films
investigated.Comment: 10 pages, 12 figures, submitted to Physical Review
Non-monotonic variation with salt concentration of the second virial coefficient in protein solutions
The osmotic virial coefficient of globular protein solutions is
calculated as a function of added salt concentration at fixed pH by computer
simulations of the ``primitive model''. The salt and counter-ions as well as a
discrete charge pattern on the protein surface are explicitly incorporated. For
parameters roughly corresponding to lysozyme, we find that first
decreases with added salt concentration up to a threshold concentration, then
increases to a maximum, and then decreases again upon further raising the ionic
strength. Our studies demonstrate that the existence of a discrete charge
pattern on the protein surface profoundly influences the effective interactions
and that non-linear Poisson Boltzmann and Derjaguin-Landau-Verwey-Overbeek
(DLVO) theory fail for large ionic strength. The observed non-monotonicity of
is compared to experiments. Implications for protein crystallization are
discussed.Comment: 43 pages, including 17 figure
Evidence of Color Coherence Effects in W+jets Events from ppbar Collisions at sqrt(s) = 1.8 TeV
We report the results of a study of color coherence effects in ppbar
collisions based on data collected by the D0 detector during the 1994-1995 run
of the Fermilab Tevatron Collider, at a center of mass energy sqrt(s) = 1.8
TeV. Initial-to-final state color interference effects are studied by examining
particle distribution patterns in events with a W boson and at least one jet.
The data are compared to Monte Carlo simulations with different color coherence
implementations and to an analytic modified-leading-logarithm perturbative
calculation based on the local parton-hadron duality hypothesis.Comment: 13 pages, 6 figures. Submitted to Physics Letters
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