960 research outputs found
Propagation of time-truncated Airy-type pulses in media with quadratic and cubic dispersion
In this paper, we describe analytically the propagation of Airy-type pulses
truncated by a finite-time aperture when second and third order dispersion
effects are considered. The mathematical method presented here, based on the
superposition of exponentially truncated Airy pulses, is very effective,
allowing us to avoid the use of time-consuming numerical simulations. We
analyze the behavior of the time truncated Ideal-Airy pulse and also the
interesting case of a time truncated Airy pulse with a "defect" in its initial
profile, which reveals the self-healing property of this kind of pulse
solution.Comment: 9 pages. 5 figure
Absorption and Emission in quantum dots: Fermi surface effects of Anderson excitons
Recent experiments measuring the emission of exciton recombination in a
self-organized single quantum dot (QD) have revealed that novel effects occur
when the wetting layer surrounding the QD becomes filled with electrons,
because the resulting Fermi sea can hybridize with the local electron levels on
the dot. Motivated by these experiments, we study an extended Anderson model,
which describes a local conduction band level coupled to a Fermi sea, but also
includes a local valence band level. We are interested, in particular, on how
many-body correlations resulting from the presence of the Fermi sea affect the
absorption and emission spectra. Using Wilson's numerical renormalization group
method, we calculate the zero-temperature absorption (emission) spectrum of a
QD which starts from (ends up in) a strongly correlated Kondo ground state. We
predict two features: Firstly, we find that the spectrum shows a power law
divergence close to the threshold, with an exponent that can be understood by
analogy to the well-known X-ray edge absorption problem. Secondly, the
threshold energy - below which no photon is absorbed (above which no
photon is emitted) - shows a marked, monotonic shift as a function of the
exciton binding energy Comment: 10 pages, 9 figure
Cholinoceptor Activation Subserving the Effects of Interferon Gamma on the Contractility of Rat Ileum
Recombinant rat interferon γ stimulated the contractility of
isolated rat ileum at doses of 4–12 units/ml. Muscarinic
cholinoceptors were involved, as treatment of the tissue with
atropine prevented the contractile response of the ileum.
Furthermore, interferon γ increased the affinity of carbachol for
the cholinoceptors and did not change its maximum effect. Neurogenic
pathways were also involved since pretreatment of ileum with
hexamethonium, hemicholinium or tetrodotoxin impaired the
contractile effect of interferon γ. In contrast to the action of
exogenous carbachol, the effects of interferon γ are indirect. They
appear to involve a G protein regulating phosphoinositide turnover
and cytoskeletal structures since they could not be induced in ileum
strips that were pretreated with pertussis toxin, phospholipase C
inhibitors (2-nitro-carboxyphenyl, NN-diphenyl carbamate and
neomycin), cytochalasine B or colchicine
Homogeneous and Narrow Bandwidth of Spike Initiation in Rat L1 Cortical Interneurons
The cortical layer 1 (L1) contains a population of GABAergic interneurons, considered a key component of information integration, processing, and relaying in neocortical networks. In fact, L1 interneurons combine top\u2013down information with feed-forward sensory inputs in layer 2/3 and 5 pyramidal cells (PCs), while filtering their incoming signals. Despite the importance of L1 for network emerging phenomena, little is known on the dynamics of the spike initiation and the encoding properties of its neurons. Using acute brain tissue slices from the rat neocortex, combined with the analysis of an existing database of model neurons, we investigated the dynamical transfer properties of these cells by sampling an entire population of known \u201celectrical classes\u201d and comparing experiments and model predictions. We found the bandwidth of spike initiation to be significantly narrower than in L2/3 and 5 PCs, with values below 100 cycle/s, but without significant heterogeneity in the cell response properties across distinct electrical types. The upper limit of the neuronal bandwidth was significantly correlated to the mean firing rate, as anticipated from theoretical studies but not reported for PCs. At high spectral frequencies, the magnitude of the neuronal response attenuated as a power-law, with an exponent significantly smaller than what was reported for pyramidal neurons and reminiscent of the dynamics of a \u201cleaky\u201d integrate-and-fire model of spike initiation. Finally, most of our in vitro results matched quantitatively the numerical simulations of the models as a further contribution to independently validate the models against novel experimental data
Dislocation Structure And Mobility In Hcp He 4
Using path-integral Monte Carlo simulations, we assess the core structure and mobility of the screw and edge basal-plane dislocations in hcp He4. Our findings provide key insights into recent interpretations of giant plasticity and mass flow junction experiments. First, both dislocations are dissociated into nonsuperfluid Shockley partial dislocations separated by ribbons of stacking fault, suggesting that they are unlikely to act as one-dimensional channels that may display Lüttinger-liquid-like behavior. Second, the centroid positions of the partial cores are found to fluctuate substantially, even in the absence of applied shear stresses. This implies that the lattice resistance to motion of the partial dislocations is negligible, consistent with the recent experimental observations of giant plasticity. Further results indicate that both the structure of the partial cores and the zero-point fluctuations play a role in this extreme mobility. © 2016 American Physical Society.117
A renormalization-group analysis of the interacting resonant level model at finite bias: Generic analytic study of static properties and quench dynamics
Using a real-time renormalization group method we study the minimal model of
a quantum dot dominated by charge fluctuations, the two-lead interacting
resonant level model, at finite bias voltage. We develop a set of RG equations
to treat the case of weak and strong charge fluctuations, together with the
determination of power-law exponents up to second order in the Coulomb
interaction. We derive analytic expressions for the charge susceptibility, the
steady-state current and the conductance in the situation of arbitrary system
parameters, in particular away from the particle-hole symmetric point and for
asymmetric Coulomb interactions. In the generic asymmetric situation we find
that power laws can be observed for the current only as function of the level
position (gate voltage) but not as function of the voltage. Furthermore, we
study the quench dynamics after a sudden switch-on of the level-lead couplings.
The time evolution of the dot occupation and current is governed by exponential
relaxation accompanied by voltage-dependent oscillations and characteristic
algebraic decay.Comment: 24 pages, 13 figures; revised versio
Influence of gas flow rate on liquid distribution in trickle-beds using perforated plates as liquid distributors
Two wire mesh tomography devices and a liquid collector were used to study
the influence of the gas flow rate on liquid distribution when fluids
distribution on top of the reactor is ensured by a perforated plate. In
opposition to most of the studies realized by other authors, conditions in
which the gas has a negative impact in liquid distribution were evidenced.
Indeed, the obtained results show that the influence of gas flow rate depends
on the quality of the initial distribution, as the gas forces the liquid to
"respect" the distribution imposed at the top of the reactor. Finally, a
comparison between the two measuring techniques shows the limitations of the
liquid collector and the improper conclusions to which its use could lead
Ghajn Klieb, (Rabat, Malta)
Between October and December 1999 a team of local
and foreign undergraduates from the University of Malta
carried out a survey of the site at Ghajn Klieb outside
Rabat. The exercise constituted the practical part of a
unit on the Principles of Archaeological Surveying
directed by Dr Nicholas Vella of the Department of
Classics & Archaeology. For the survey the team was
joined by Hanna Stager, a graduate of the same
department, who also researched some of the references
used in this article. Initial reconnaissance of the site was
carried out on 15 October 1999 with Nathaniel Cutajar
and Michelle B uhagiar, Curator and Assistant Curator
respectively at the National Museum of Archaeology.
The scatter of surface ceramics and the existence of
previously known and unknown features revealed the
extent and potential of the site. It was decided that the
locality of Ghajn Klieb warranted systematic study that
could be carried out in various stages, with the longterm
aim being an assessment of human activity and
cultural behaviour at the site. The Museums Department
gave the go-ahead for this project, and permission to
collect the surface ceramics was granted. This short
report is intended to give an outline of the work
undertaken to date. Emphasis is placed on the field
methods adopted and on the presentation of what we
believe to be worth talking about at this stage. An effort
is here made by the senior author to unravel the
collaborative nature of the exercise by lending weight
to individual thoughts and interpretations that arose while
work progressed in the field.peer-reviewe
Ideating IDNA: Lessons and Limitations From Leeches in Legacy Collections
Indirect methods for conducting faunal inventories present great promise, and genomic inventories derived from environmental sources (eDNA) are improving. Invertebrate ingested DNA (iDNA) from terrestrial leeches in the family Haemadipsidae has shown potential for surveying vertebrates and biodiversity monitoring in protected areas. Here we present an initial, and critical, evaluation of the limitations and biases of current iDNA protocols for biodiversity monitoring using both standard and NGS barcoding approaches. Key findings include the need for taxon relevant multi-locus markers and reference databases. In particular, the limitations of available reference databases have profound potential to mislead and bias eDNA and iDNA results if not critically interpreted. Nevertheless, there is great potential for recovery of amplifiable DNA from gut contents of invertebrate museum specimens which may reveal both temporal patterns and cryptic diversity in protected areas with increased efficiency. Our analyses of ingested DNA (iDNA) from both freshly stored and previously collected (legacy) samples of terrestrial leeches successfully identified vertebrates from Myanmar, Australia and Madagascar and indicate the potential to characterize microbial communities, pathogen diversity and interactions at low cost
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