40,132 research outputs found
Destruction of Refractory Carbon in Protoplanetary Disks
The Earth and other rocky bodies in the inner solar system contain
significantly less carbon than the primordial materials that seeded their
formation. These carbon-poor objects include the parent bodies of primitive
meteorites, suggesting that at least one process responsible for solid-phase
carbon depletion was active prior to the early stages of planet formation.
Potential mechanisms include the erosion of carbonaceous materials by photons
or atomic oxygen in the surface layers of the protoplanetary disk. Under
photochemically generated favorable conditions, these reactions can deplete the
near-surface abundance of carbon grains and polycyclic aromatic hydrocarbons by
several orders of magnitude on short timescales relative to the lifetime of the
disk out to radii of ~20-100+ au from the central star depending on the form of
refractory carbon present. Due to the reliance of destruction mechanisms on a
high influx of photons, the extent of refractory carbon depletion is quite
sensitive to the disk's internal radiation field. Dust transport within the
disk is required to affect the composition of the midplane. In our current
model of a passive, constant-alpha disk, where alpha = 0.01, carbon grains can
be turbulently lofted into the destructive surface layers and depleted out to
radii of ~3-10 au for 0.1-1 um grains. Smaller grains can be cleared out of the
planet-forming region completely. Destruction may be more effective in an
actively accreting disk or when considering individual grain trajectories in
non-idealized disks.Comment: 15 pages, 9 figures, Accepted for publication in Ap
Anisotropy Control in Photoelectron Spectra: A Coherent Two-Pulse Interference Strategy
Coherence among rotational ion channels during photoionization is exploited
to control the anisotropy of the resulting photoelectron angular distributions
at specific photoelectron energies. The strategy refers to a robust and single
parameter control using two ultra-short light pulses delayed in time. The first
pulse prepares a superposition of a few ion rotational states, whereas the
second pulse serves as a probe that gives access to a control of the molecular
asymmetry parameter for individual rotational channels. This is
achieved by tuning the time delay between the pulses leading to channel
interferences that can be turned from constructive to destructive. The
illustrative example is the ionization of the state of
Li. Quantum wave packet evolutions are conducted including both
electronic and nuclear degrees of freedom to reach angle-resolved photoelectron
spectra. A simple interference model based on coherent phase accumulation
during the field-free dynamics between the two pulses is precisely exploited to
control the photoelectron angular distributions from almost isotropic, to
marked anisotropic
Ultra-compact branchless plasmonic interferometers
Miniaturization of functional optical devices and circuits is a key
prerequisite for a myriad of applications ranging from biosensing to quantum
information processing. This development has considerably been spurred by rapid
developments within plasmonics exploiting its unprecedented ability to squeeze
light into subwavelength scale. In this study, we investigate on-chip plasmonic
systems allowing for synchronous excitation of multiple inputs and examine the
interference between two adjacent excited channels. We present a branchless
interferometer consisting of two parallel plasmonic waveguides that can be
either selectively or coherently excited via ultra-compact antenna couplers.
The total coupling efficiency is quantitatively characterized in a systematic
manner and shown to exceed 15% for small waveguide separations, with the power
distribution between the two waveguides being efficiently and dynamically
shaped by adjusting the incident beam position. The presented design principle
can readily be extended to other configurations, giving new perspectives for
highly dense integrated plasmonic circuitry, optoelectronic devices, and
sensing applications.Comment: 15 pages, 6 figure
The given and a proximity to art: Heidegger's early dialectical conception of phenomenology
Accepted manuscrip
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Endoplasmic reticulum stress, degeneration of pancreatic islet β-cells, and therapeutic modulation of the unfolded protein response in diabetes.
BackgroundMyriad challenges to the proper folding and structural maturation of secretory pathway client proteins in the endoplasmic reticulum (ER) - a condition referred to as "ER stress" - activate intracellular signaling pathways termed the unfolded protein response (UPR).Scope of reviewThrough executing transcriptional and translational programs the UPR restores homeostasis in those cells experiencing manageable levels of ER stress. But the UPR also actively triggers cell degeneration and apoptosis in those cells that are encountering ER stress levels that exceed irremediable thresholds. Thus, UPR outputs are "double-edged". In pancreatic islet β-cells, numerous genetic mutations affecting the balance between these opposing UPR functions cause diabetes mellitus in both rodents and humans, amply demonstrating the principle that the UPR is critical for the proper functioning and survival of the cell.Major conclusionsSpecifically, we have found that the UPR master regulator IRE1α kinase/endoribonuclease (RNase) triggers apoptosis, β-cell degeneration, and diabetes, when ER stress reaches critical levels. Based on these mechanistic findings, we find that novel small molecule compounds that inhibit IRE1α during such "terminal" UPR signaling can spare ER stressed β-cells from death, perhaps affording future opportunities to test new drug candidates for disease modification in patients suffering from diabetes
Pathogenetic sequences in coeliac disease : closing the jigsaw puzzle
Coeliac disease is one of the best understood models of adaptive immunity in which a known dietary component triggers over-presentation of a known autoantigen in genetically predisposed individuals. The dynamics of this gene-nutrient interaction and the mechanism responsible for accelerated programmed death of the enterocytes lining the upper small intestine are explored in order to generate insight into the large number of candidate pharmacological agents that may well aid or replace cumbersome dietary treatment in years to come.peer-reviewe
Global coherence of quantum evolutions based on decoherent histories: theory and application to photosynthetic quantum energy transport
Assessing the role of interference in natural and artificial quantum
dyanamical processes is a crucial task in quantum information theory. To this
aim, an appopriate formalism is provided by the decoherent histories framework.
While this approach has been deeply explored from different theoretical
perspectives, it still lacks of a comprehensive set of tools able to concisely
quantify the amount of coherence developed by a given dynamics. In this paper
we introduce and test different measures of the (average) coherence present in
dissipative (Markovian) quantum evolutions, at various time scales and for
different levels of environmentally induced decoherence. In order to show the
effectiveness of the introduced tools, we apply them to a paradigmatic quantum
process where the role of coherence is being hotly debated: exciton transport
in photosynthetic complexes. To spot out the essential features that may
determine the performance of the transport we focus on a relevant trimeric
subunit of the FMO complex and we use a simplified (Haken-Strobl) model for the
system-bath interaction. Our analysis illustrates how the high efficiency of
environmentally assisted transport can be traced back to a quantum recoil
avoiding effect on the exciton dynamics, that preserves and sustains the
benefits of the initial fast quantum delocalization of the exciton over the
network. Indeed, for intermediate levels of decoherence, the bath is seen to
selectively kill the negative interference between different exciton pathways,
while retaining the initial positive one. The concepts and tools here developed
show how the decoherent histories approach can be used to quantify the relation
between coherence and efficiency in quantum dynamical processes.Comment: 13 papges, 9 figure
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