2,584 research outputs found
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
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
Evidence of polariton induced transparency in a single organic quantum wire
The resonant interaction between quasi-one dimensional excitons and photons
is investigated. For a single isolated organic quantum wire, embedded in its
single crystal monomer matrix, the strong exciton-photon coupling regime is
reached. This is evidenced by the suppression of the resonant excitonic
absorption arising when the system eigenstate is a polariton. These
observations demonstrate that the resonant excitonic absorption in a
semiconductor can be understood in terms of a balance between the exciton
coherence time and the Rabi period between exciton-like and photon-like states
of the polariton.Comment: 9 pages and 4 figure
Sustaining Collection Value: Managing Collection/Item Metadata Relationships
Many aspects of managing collection/item metadata relationships are critical to sustaining collection value over time. Metadata at the collection-level not only provides context for finding, understanding, and using the items in the collection, but is often essential to the particular research and scholarly activities the collection is designed to support. Contemporary retrieval systems, which search across collections, usually ignore collection level metadata. Alternative approaches, informed by collection-level information, will require an understanding of the various kinds of relationships that can obtain between collection-level and item-level metadata. This paper outlines the problem and describes a project that is developing a logic-based framework for classifying collection-level/item-level metadata relationships. This framework will support (i) metadata specification developers defining metadata elements, (ii) metadata librarians describing objects, and (iii) system designers implementing systems that help users take advantage of collection-level metadata.Institute for Museum and Libary Services (Grant #LG06070020)published or submitted for publicationis peer reviewe
Dynamic Phase Transitions in Cell Spreading
We monitored isotropic spreading of mouse embryonic fibroblasts on
fibronectin-coated substrates. Cell adhesion area versus time was measured via
total internal reflection fluorescence microscopy. Spreading proceeds in
well-defined phases. We found a power-law area growth with distinct exponents
a_i in three sequential phases, which we denote basal (a_1=0.4+-0.2), continous
(a_2=1.6+-0.9) and contractile (a_3=0.3+-0.2) spreading. High resolution
differential interference contrast microscopy was used to characterize local
membrane dynamics at the spreading front. Fourier power spectra of membrane
velocity reveal the sudden development of periodic membrane retractions at the
transition from continous to contractile spreading. We propose that the
classification of cell spreading into phases with distinct functional
characteristics and protein activity patterns serves as a paradigm for a
general program of a phase classification of cellular phenotype. Biological
variability is drastically reduced when only the corresponding phases are used
for comparison across species/different cell lines.Comment: 4 pages, 5 figure
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