3,251 research outputs found
Role of Fermion Exchanges in Statistical Signatures of Composite Bosons
We study statistical signatures of composite bosons made of two fermions
using a new many-body approach. Extending number-states to composite bosons,
two-particle correlations as well as the dispersion of the probability
distribution are analyzed. We show that the particle composite nature reduces
the anti-bunching effect predicted for elementary bosons. Furthermore, the
probability distribution exhibits a dispersion which is greater for composite
bosons than for elementary bosons. This dispersion corresponds to the one of
sub-Poissonian processes, as for a quantum state, but, unlike its elementary
boson counterpart, it is not minimum. In general, our work shows that it is
necessary to take into account the Pauli exclusion principle which takes place
between fermionic components of composite bosons - along the line here used -
to possibly extract statistical properties in a precise way.Comment: 14 page
Optical signatures of a fully dark exciton condensate
We propose optical means to reveal the presence of a dark exciton condensate
that does not yield any photoluminescence at all. We show that (i) the dark
exciton density can be obtained from the blueshift of the excitonic absorption
line induced by dark excitons; (ii) the polarization of the dark condensate can
be deduced from the blueshift dependence on probe photon polarization and also
from Faraday effect, linearly polarized dark excitons leaving unaffected the
polarization plane of an unabsorbed photon beam. These effects result from
carrier exchanges between dark and bright excitons.Comment: 5 pages, 4 figure
Effects of fermion exchanges on the polarization of exciton condensates
Exchange processes are responsible for the stability of elementary boson
condensates with respect to their possible fragmentation. This remains true for
composite bosons when single fermion exchanges are included but spin degrees of
freedom are ignored. We here show that their inclusion can produce a
"spin-fragmentation" of a condensate of dark excitons, i.e., an unpolarized
condensate with equal amount of dark excitons with spins (+2) and (-2). Quite
surprisingly, for spatially indirect excitons of semiconductor bilayers, we
predict that the condensate polarization can switch from unpolarized to fully
polarized, depending on the distance between the layers confining electrons and
holes. Remarkably, the threshold distance associated to this switching lies in
the regime where experiments are nowadays carried out.Comment: 5 pages, 1 figur
Environmental analysis of the chemical release module
The environmental analysis of the Chemical Release Module (a free flying spacecraft deployed from the space shuttle to perform chemical release experiments) is reviewed. Considerations of possible effects of the injectants on human health, ionosphere, weather, ground based optical astronomical observations, and satellite operations are included. It is concluded that no deleterious environmental effects of widespread or long lasting nature are anticipated from chemical releases in the upper atmosphere of the type indicated for the program
The exciton many-body theory extended to arbitrary composite bosons
We have recently constructed a many-body theory for composite excitons, in
which the possible carrier exchanges between excitons can be treated
exactly through a set of dimensionless ``Pauli scatterings'' between two
excitons. Many-body effects with excitons turn out to be rather simple because
excitons are the exact one-electron-hole-pair eigenstates of the semiconductor
Hamiltonian, thus forming a complete orthogonal set for one-pair states. It can
however be of interest to extend this new many-body theory to more complicated
composite bosons, \emph{i. e.}, ``cobosons'', which are not necessarily the
one-pair eigenstates of the system Hamiltonian, nor even orthogonal. The
purpose of this paper is to derive the ``Pauli scatterings'' and the
``interaction scatterings'' of these cobosons formally, \emph{i. e.}, just in
terms of their wave functions and the interaction potentials which exist
between the fermions from which they are constructed. We also explain how to
derive many-body effects in this very general system of composite bosons
Degenerate mixing of plasma waves on cold, magnetized single-species plasmas
In the cold-fluid dispersion relation ω = ω_p/[1+(k_⊥/k_z)^(2]1/2) for Trivelpiece-Gould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination k_⊥/k_z. As a result, for any frequency ω<ω_p, there are infinitely many degenerate waves, all having the same value of k_⊥/k_z. On a cold finite-length plasma column, these degenerate waves reflect into one another at the ends; thus, each standing-wave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the many-wave modes is that the short-wavelength waves often add constructively along resonance cones given by dz/dr = ±(ω_p^2/ω^2-1)^(1/2). Also, the presence of short wavelengths in the admixture for a predominantly long-wavelength mode enhances the viscous damping beyond what the single-wave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of low-order, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid
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